A series that might be too gentle after the eighth hack:
5 * 1.25^(#hacks -1)
5 6 8 10 12 15 19 24. That also comes up to around 100 total, however..
9 10 11 12
51 77 115 173 old
30 37 47 58 my example
Things don't get quite prohibitive enough.
If you want to mess around with the equations yourself, here's an easy little Open Office Spreadsheet. It is a close enough approximation, just change the base and power in the third row. It was hastily put together, but it should let you experiment with exponential growth quite quickly.
If you want to have a higher base cost - and things get prohibitive after the eighth hack, you're going to have to complicate the equation a bit further.
You could keep the cost relatively low but multiply it by the mark of the planet in question..
Anyways, I have no opinion other than if you want a good curve you might have to have two sets of equations, F(x) when x<=8, and F(x) when x>8.
or something like that.