Are you referring to the Illinois Schedule 1299 I Income Tax Credits Information and Worksheets ( Column F)?
Per instruction, the basis is the purchase price plus any capital expenditures minus any rebates and IRC Section 179 deductions.
The basis is not reduced by depreciation, including bonus depreciation, except the depreciation you were allowed to claim before the date you placed it in service in Illinois or in an Illinois enterprise or river edge redevelopment zone.
For more information, click here Basis page 5.