Right now troop transport modules costs are as summarized below. Generally the progression between sizes seems reasonable, and as you'd expect the capacity/cost ratio goes up with larger sized modules. However, even if you build the largest sized drop bays, you're looking at a cost of 32k BP to be able to build just the drop modules to transport 1M troops to an enemy world. That's a huge amount of capacity to build through a shipyard, and still a pretty incredible lift for even the entire economy of a largeish planet. In 2055 my capital has only 21k MP per year. Now granted I've moved a significant chunk of factories to colonies, but still ~half are on Earth.
Once you factor in engines and some decent armor, you're looking at ~12k BP per 100k of troops transported, so 120k BP for 1M troops you can drop on a hostile NPC capital world while under STO fire. It strikes me as pretty steep. I'm going to be building drop ships for ~10+ years, and that's accounting for building some components via industry rather than at the shipyard. In hindsight I should've started building Very Large Troop Transport Drop Bays at a low rate from the start of the game, such that I was ready to build them into ships in a shipyard once I found a tempting NPC homeworld. That's certainly what I'll do next time. That said, are troop drop bay modules unreasonably expensive, or did I just wait too long to start building them?
Troop Transport Bay Size Capacity Cost
Very Small 100 100 3
Small 250 250 6
Standard 1,000 1,000 20
Large 5,000 5,000 80
Very Large 25,000 25,000 320
Troop Transport Drop Bay Size Capacity Cost
Very Small 120 100 8
Small 300 250 15
Standard 1,200 1,000 50
Large 6,000 5,000 200
Very Large 30,000 25,000 800