it would also make full country rebuilds much more expensive

anyway to put it in a spreadsheet:

=MAX(.2;(1-10*CS/EMPTY_LAND))

change the comma to a semi colon

then what you get is a formula where for every 1k acres you have unbuilt, your building cost is reduced by 80% if you have 80 csites

400 csites needed if you have 5k unbuilt and so on

=(1500+3*EMPTY_LAND)

And for the first part your building cost would be 4500 if you had 1k acres unbuilt

1k = 4500
2k = 7500
3k = 10500

and so on

So at the right csites count 1k acres would only ever cost 900 each to build or 900k cash. Which just doesn't sound great to me.