I have been trying to track how much it cost to add levels and I think it has less to do with the number of levels but the mix of levels between commerical, residential and imperial. I think the formula is something like:
Base Cost + (7 x Number of Comercial Levels) - (1 x Number of Residential & Imperial levels) = Unlock Cost.
Need more data to confirm and to back out the Base Cost for each level. I am also not possitive on the effect of Imperial levels. ANy additional observations would be helpful.