I don’t think it’s a good idea to lower the cost of air or naval bases. I know they are a bit pricy at $15, but that’s still not too bad considering how much they could help you, particularly in certain strategies. After all, a carrier is only $1 more and I bet we all buy a lot of these and they stand the chance of getting destroyed. At least bases are permanent. Sure an enemy could capture them, but you could capture them back. Plus, when you buy an air or naval base, all your Allies can use it as well.
In general, I think most people buy an air or naval base for a specific purpose (Japan India crush, protecting transports off Romania, strengthening Gibraltar or Egypt, etc.). I don’t think it would be right to make it easier (ie. cheaper) to accomplish those goals. If you are going to decide on a certain course of action, you should have to really commit to it at full price, whether it be buying bases, ICs or specific units to accomplish it.
Also, I just think Air and Naval bases provide too many advantages to cut the price down so. If they were so cheap, it seems to me like there would be bases all over the board.