Just pulling a number out of my butt... around 5k.
Let's see..
computer ~600
3 monitors ~200x3
Racing controls ~400
seat ~100
structure ~300
linear actuators ~300 x 4
control board ~200
Ok so that totals around 3400, but I've got to think there would be tons of hidden costs I can't think of. This is to buy the stuff and assemble it mind you, not to make it from scratch, which for some of the stuff (structure, controls and control board) would be a lot cheaper.