Logistics

First of all, one gets to build a certain number of starbases (the number dependent on map size) for free.

Ok that's say you a play a medium sized map. The cost is dependent on the number starbases in your possession.

# of SB possessed......cost(BC)to build next SB
0.............................0
1.............................0
2.............................0
3.............................0
4.............................0
5............................150
6............................300
7............................450
8............................600
and so on...

Second the cost is modified by your logistics ability. The above numbers are based on 0 logistics ability. Let's say you build the first 2 logistcs improvements. They would give you a total logistics of 40. The costs
would increase by 90 BC[which is 150-(0.40*150)]instead of by 150 BC.

So if you have 8 SBs, the 9th will cost 360BC instead of 600 BC.

Third, some starbase modules can be built for free while others cost BCs to build. I don't have the up to date list written down.
[Message Edited]

Thanks, Moser! Much more clear now.