the TL9 plant is 3× the size, and the same cost per ton as a TL15; even if it's the only TL15 item aboard, a merchant 200 Td J1 M1 P1 saves 4 Td, and MCr12... 50,000 a month in payment savings AND increasing payload by 4 Td, essentially either a steady Cr3600 in freight (Allowing for typical 90% fill) or 4k after expenses for a stateroom (including increased payment).
Nothing else a J1 M1 merchant needs decreases in cost per rating as radically, and the PP on a 200 Td merchantman is MCr 8 under Bk2; under HG, a TL9 is 3%, at MCr3 per Td; a TL 15 is 1% at MCr3 per Td. So... that MCr18 vs MCr6 on that 200Td J1 M1, but no change on the others.
Even a Fuel Purification Plant (allowing use of unrefined fuel without concern for misjumps) has a lower cost/benefit return on investment than the TL=15 power plant. 50 tons of refined fuel per month (2J1) is only Cr25,000 ... not Cr50,000 like
@aramis cites.
has actually been settled for over a thousand years
The Spinward Marches may have been inhabited for over a thousand years ... but there's still a LOT of frontier out there. The sheer quantity of Non-Industrial worlds (population: 6-) far outnumbers the quantity of population: 7+ worlds.
Using Traveller Map's search feature ... using in:spinward uwp:????[0-6]*-* as the search term yielded 160 star systems, while using in:spinward uwp:????[7-8]*-* as the search term yielded 98 star systems, and using in:spinward uwp:????[9-A]*-* as the search term yielded only 22 star systems.
No surprise, low population worlds outnumber high(er) population worlds in the Spinward Marches sector.
Also, according to Traveller Wiki the
Spinward Marches sector has been settled since the 400s (so less than 700 years by 1105) and the Express Network wasn't fully built out to the sector until the early 700s ... meaning that by 1105 the sector has only been "properly integrated" with the rest of the Third Imperium for about 400 years or so. There are "pockets of civilization" scattered around the sector, but there's still PLENTY of wilderness and frontier remaining to be explored and have adventures in. It's actually a pretty decent mixture of tech and population to play with.
there are several high population TL15 worlds that can make all the TL15 goods the sector would need
The Spinward Marches has
FOUR TL=15 star systems inside the Spinward Marches ... Glisten, Rhylanor, Mora and Trin.
Of those four, only Rhylanor isn't an Industrial mainworld, while Glisten, Mora and Trin are Industrial mainworlds (in 1105), per LBB S3.
I'm not entirely convinced that three industrial worlds can produce ALL of the TL=15 good(ie)s that are there is going to be a demand for throughout the sector. In other words, I'm thinking the demand will outstrip the supply ... and you don't need to be an economics major to know what that will do to prices.
Mora may be the most powerful economic world in the sector (and the "Gateway to the Marches") but there's still going to be an upper limit on what Mora can supply to the rest of the sector (with a significant portion of that supply getting dedicated to supporting the Imperial Navy).
So TL=15 "stuff" exists in the sector ... but it isn't going to be ubiquitous, is my point.
And in places like the Five Sisters, Jewell or Vilis subsectors, the supply lines are long enough to make obtaining TL=15 goods ... unreliable at best (hence the impetus for my
Five Sisters Clipper and
Karin Corvette designs to overcome the "tyranny of distance" due to the location of the Five Sisters subsector).