as cost you use the actual original cost + cost of improvements. the lesser of the gain or depreciation allowed or allowable must be recaptured as section 1250 gain. if you did not take depreciation see a tax pro.
example
selling price 100,000
selling expenses -10,000
net 90,000
cost + improvements- 60,000
depreciation allowed or allowable -8.000
tax basis for gain 52,000
gain (90K-52K) 38,000 of which 8K is taxed as 1250 gain and $30K is taxed as long-term capital gain