Scraping them is a good idea generally once you are done with them.

There's a logistics-based limit on how many starbases you can have - once you go over, it starts costing more and more to build each new starbase. Really, the question is why not destroy it?

It's not a limit, it's a threshold.

Your first 10 are free; every successive SB costs (SB #-10) * (150-trunc(1.5*logistics)) to build.

What this means is if you have 100 logistics, all SB constructions are free, but as you normally cap around 57/58 with a Hyperion Logistics System, 100 is rather difficult to achieve in a "normal" game.

Perhaps a more useful piece of information is prices at 58 logistics:
150-(1.5*58)=150-87=63bc

1st SB = 63BC, by itself
11th SB = 693BC, by itself
100th SB = 6300BC, by itself

1st through 11th SBs = sum(1,...,11)*cost = (11+1)*(11/2) or 11*(11+1)/2
11*12/2=66*cost = 66*63=4,158BC

1st through 100th SBs = (100+1)*(100/2) or 100*(100+1)/2
100*101/2=5050*cost = 5050*63 = 318,150BC

So it can add up fast.

MarvinKosh is correct.