Just the cost of the engine is pretty cheap, but wont be the main cost of the project. Bear in mind the computer if you dont get stock management, or redoing the loom to fit a sprinter even if you get all electrics.
Then there's the change to EFI, and all the little bolts and bits of hose and pipe that chew up the pocket-money.
Sit down and do the sums properly before you start. There should be plenty of info around here.