Cheaper for whom?
The provider (ship-owner), of course.
This is where "right sizing for routes" becomes the overriding factor.
Why overbuild capability (which is expensive!) if you aren't going to be utilizing your full capacity all the time?
Or to put it simply ... why use a J4 clipper for a 2 parsec trip? It's just wasteful and inefficient.
But this is where external cargo loading capacity comes into play!
If you have a J4 clipper ship, but your next destination is only 2 parsecs away, then your ship is the "wrong size for where you're going" (in effect) for your next trip. However, if you could add on EXTERNAL cargo loading and "tow it all" through jump space, you could
situationally increase as needed the amount of cargo capacity you can transport.
Referring to the
Optimized Drive Usage tables once more, a 600 ton hull with an LBB2 Jump-M standard drive yields:
- J4 at 600 tons (no external loading)
- J3 at 800 tons (+200 tons external load)
- J2 at 1200 tons (+600 tons external load)
- J1 at 2400 tons (+1800 tons external load)
Basic idea is no different from External Demountable Fuel Tanks ... except you're "bolting on" more cargo space, rather than just more fuel tanks.
In other words, a 600 ton J4 clipper ship can
tow through jump 1/3/9 Far Traders as external cargo 
if necessary by accepting a "range penalty" for the privilege of doing so.
In a LBB5.80 compliant TU, that "external towing capacity" doesn't come for free (it's almost cost free, but not completely cost free).
LBB5.80, p32:
Small Craft: Various non-starships (such as pinnaces, cutters, ship's boats, shuttles, lifeboats, and fighters) are detailed in the section on small craft. Small craft are carried at their own tonnage on ships 1000 tons and under; they require tonnage equal to 130% of their mass within the hull of larger ships. The cost is Cr2,000 per ton.
Big Craft: Ships may also carry non-starships greater than 99 tons, or even other starships, provided proper arrangements are made. Big craft require tonnage equal to 110%of their mass in the ship; the cost is Cr2,000 per ton.
Vehicle Launch Facilities: Starships and non-starships carried on a ship must be provided with some form of launch facilities.
1. Dispersed Structures: Ships which have a type 7 configuration hull carry craft and ships attached to their exterior. They need no additional fittings. All craft carried by a configuration 7 ship may be launched in one turn.
2. Launch Facility: Ordinary launch facilities for a ship allow one craft to be launched per turn per 10,000 tons of hull. These facilities are available at no cost or additional tonnage.
The basic takeaway is ... if you want to build an external towing capacity into a small craft/non-starship/starship, doing so will cost
Cr2000 per ton of external towing capacity.
So if you want a 600 ton hull with an LBB2 Jump-M standard drive to have up to 1800 tons of external load capacity, that is going to require:
2000 * 1800 = MCr3.6 additional base construction cost
Now, presumably a craft (small/big/star) cannot retain any kind of streamlining "advantage" while it is encumbered with external loads ... such as an array of
Modular Cutter Modules or needing to dock with a 200 ton Free/Far Trader (or a few of them) for interstellar transport ... so I would stipulate that just like with External Demountable Fuel Tanks (LBB A5, p14), any craft towing an external load of any quantity is considered unstreamlined regardless of its native configuration. Only in a "clean, unencumbered by external loads" condition can a craft be considered either streamlined or partially streamlined as per its constructed configuration.
So by that standard ... a stock 200 ton Jump-B drive Far Trader has 61 tons of internal cargo space @ J2 ... and up to +200 tons of external cargo capacity @ J1 ... at an additional expense of MCr0.4 for the modifications to the ship's hull (200*2000=MCr0.4).
A 600 ton Jump-M drive J4 clipper could also have ~60 tons of internal cargo space @ J4 ... and up to +1800 tons of external cargo capacity @ J1 ... at an additional expense of MCr3.6 for the modifications to the ship's hull (1800*2000=MCr3.6).
3x the (primary) hull displacement.
2x the (base) jump range.
9x the external load capacity. 
Now, as a Referee ... the way I would rule that the external load capacity for a ship can be filled would be by either:
- Major Cargo ONLY (in increments of 10 ton lots in CT) bound for a starship's declared destination.
- Charter of the external load capacity available for the starship's declared destination.
For option 1 (major cargo only), the major cargo is assumed to be inside containers which can survive exposure to vacuum and which can be towed through jumpspace close to the hull of the starship (within a jump bubble, basically). This option yields a base Cr1000 per ton (just like regular internal cargo rates) because the starship operator is contracting for the transport directly.
For option 2 (charter), the starship operator basically "sells" the rights to the ship's external load capacity (consistent with the declared destination) to
an interested 3rd party ... who then delivers the containers of cargo for transfer to the starship to be loaded externally. With the charter option, it becomes
someone else's responsibility to scare up enough cargo to fill up the charter capacity, regardless of how much (major) cargo was (or was not) available to the starship operator to transport. This option yields a base Cr900 per ton per 2 week time block for interstellar cargo deliveries.
Note that if the declared destination is an Amber Zone, there will be no major cargo available to a starship operator for transport ... so option 1 could be "right out" in that case. However, a 3rd party may still have amassed a substantial quantity of cargo that needs to be transported to the Amber Zone world (hence why it was not available to the starship operator in the first place!) and that 3rd party is essentially working as a fixer/broker on behalf of clients wanting to transport that cargo to the starship's declared destination. The 3rd party would be billing Cr1000 per ton to their clients, but only paying Cr900 to the starship operators for chartered transport services. Presumably the 3rd party would have capital investments in the necessary containerization to facilitate such operations ... they just need starships to move the containers.
And it's through charters with 3rd parties (at Cr900 per ton), rather than the starship operator contracting directly with clients for cargo freight transport (at Cr1000 per ton), that it is possible to "get around" certain trade restrictions (such as Amber Zones having no major cargo bound for them).
As for whether such charters are available to a starship's declared destination ... I'm thinking:
- Add the world population value of the origin and destination worlds then divide by 2 (round up).
- DMs (origin and destination): +1 per type B starport, +2 per type A starport
- Roll the threshold or less on 2D6 for there to be a charter available to the starship's declared destination.
So a J1 transit from Rhylanor/Rhylanor to Porozolo/Rhylanor, for example, would involve a population 9 and A (so average is 10) and two type A starports (so +4 DM). Roll 14- on 2D6 for there to be charter available for pickup that will max out the starship's manifest.
And it's at this point, with external loading and charters that a wily starship operator can start self dealing on speculative cargoes.
A savvy operator would charter their external load capacity (only) to their declared destination ... but keep their internal cargo hold available for speculative cargo and direct contracts (at Cr1000 per ton), with charters for the internal capacity as a last resort if they can't fill their internal cargo hold.
And
*THAT* is how you make a profit with a J3+ clipper ship!
