Well ok, lets assume you are new

Every planet has a limited amount of spaces.

For example a planet with 250 spaces such as your home planet. This planet already has 10 buildings on it.

So 240 open spots left. You have space to build 250 though. But you will have to pay extra for those 10. This extra cost is due to OVERBUILDING.

If you have for example a planet with 250 buildings on it and it has 250 open spaces this planet is full built. (this they call fb)

If you want to add more buildings on this planet you will have to pay more... so if you are fb you are paying more for each building (last example it were only 10 of them).

This OB is in %. Sometimes people have planets up to 1000%-1500% OB. But then you pay alot more. If you're 300% ob you pay 4 times as much. 400% Ob = 5 times, 500% is 6 times as much, etc.

Lol edit haha

" its due to rounding and because the game sucks" Someone said ;p

Ok lol. 250 would never be possible, but 249 yes.