New 4 family development in metro Milwaukee, Wisconsin. Cost??

So I have the opportunity to go in on this development project North of Milwaukee, less than a mile from major Expressway, also neighboring a small wooded/grassy park area. Right now it is currently zoned for 2, 36 unit apartment buildings, 8-10(?) acres but if i were to go forward on the development i would be much more inclined to rezone to 4 family Condominiums if i could. Right now it is basically just an unused field that has been for sale for a number of years. The entire reason I’m interested in this property is because the owner of the land is willing to provide the land to me, free of cost, and i have to develop it, and get as much out of it as i can with us obviously working out a deal with profit percentages in the end. I have developed 2 side by side duplex condominiums earlier this year, which went smoothly, but this is a much larger project and am basically just looking for pointers, cost estimates on 4 family slab on grade vs my side by sides w basements. Thanks in advance !!