The other possibility is that they fire 1 missile.
Think of them as torpedo launchers rather then missile turret arrays. Possibly with Multiple heads. It then leaves the fact that these missiles are not listed for price. 250kCr sounds about right (the same cost as 50 missiles)
Talking of 50 missiles, that is about how effective a bay is. Because the damage of bay weapons is moderated by TL, rather then by number of missiles it is a little hard to determine, however at TL7, a 100 ton bay is slightly more effective then an array of 10 tripple turrets (USP 7 vs USP 6), at TL 13 the 100ton bay is USP 9, the 50 ton bay is at USP 8, and the tripple turret array is at USP 7. In MTU I am allowing the 100ton bay to carry many more ammo internally, but otherwise the array is the same size. The difference in effectiveness comes from additional guidance and communications systems.
50 ton missile bay =
20 ton launch array
10 ton crew guidance/maintenance area
20 ton missile storage
100 ton missile bay =
20 ton launch array
20 ton crew guidance/maintenance area
60 ton missile storage
With 50 missiles per shot the 50 ton bay only has 9 missile launches before reloading (and with that number of missiles that will take a long time) the 100 ton bay has 25, probably enough for a protracted fight. I am not expecting any players at any time to look at a missile bay as a good idea, they whimper when firing a single missile tripple turret (15kCr... oh my!). At over 2MCr for the missiles loaded into a 50 ton bay this is an expensive weapon system that may bring a moment of pause before firing.
Getting back to the original question as far as I can tell this information is not listed. The effectiveness of the bay weapons is noted, but not what ammo is required, or how much it costs.
Lastly, another option is that the missile bay weapons have something other then missiles going for them. For example a rapid launch rail to put missiles in the water alrady travelling at high velocity. This is unlikely conceptually as the missile bays have no energy requirement, although it would cut down on the cost/shot equation, as the missiles involved would only have guidance, and not primary launch fuel meaning that you could have a much larger warhead per missile.