Pretty good work for a first mod. It works, it looks sleek, and it appears to be bug-free.
I can see gameplay purpose for having upgrades to the facilities, but it conflicts with my immersion to think that you can pay more money to squeeze more people or things into the same space. I believe that is why I and others have settled on the more realistic way of adding a large version of the facility which is more space efficient. But don't let me discourage your ideas.
I would also suggest that the upgrade costs more than you get. You have a 50% increase costing 50% more. That makes it intrinsically a better option. For example, 2 upgraded facilities costs the same amount of money as 3 basic facilities, but it costs less space. If it were a trade-off between space and money, it would then be a choice rather than a single correct answer. Early on, the player might want to expand through the available space in their base, saving money by building basic facilities. But later in the game when they run short on space and have lots of money to spend, they can easily upgrade existing facilities. Also, any player who wants to defend a small base can use the less money-efficient upgraded facilities for the express purpose of shrinking the base size and making base defense missions quicker. I would suggest at least double cost, perhaps as much as triple cost.
You could possibly lock the upgrades behind research. Maybe the aliens are using
Planck Generator or
Megafluxer technology to make things
bigger on the inside and we can steal the tech to use in our own bases. Or maybe it doesn't require research. Maybe it's just money: titanium-alloy composite structure can support the same amount of dirt as concrete-rebar structure, albeit in less space. Sometimes throwing money at the problem really does lead to marvelous solutions. Add a bit of lore into the UFOPedia and silence the outcries of nerds like me.
Speaking of the UFOPedia, you don't yet have any UFOPedia entries for the larger facilities. You could add just one article explaining that every facility has a larger version and why, or you could put in one article for every larger version, top give a bit more detail on each specific case. The UFOPedia entries are the least important part overall, but I feel it's the place where you really get to express your creativity and explain the rationale behind your decisions.
You don't have refund values set for the upgraded facilities, and the basic facility refund value is set to 100%. This means that if you dismantle a basic facility, you will get all of your money back. If you dismantle an upgraded facility, you won't get any money back.
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Last thing: about the authors -- you're the only author of the mod. ohartstein and I may have provided you some code to work with, but you put it together into a mod. You should definitely list yourself first in the author list, and probably only you. You can give us special thanks in the metadata file if you like. =)