I've seen a lot of carbed guys paying up to $1500 for a good quality job. Some have got away with it for less than $1000.
I rang Motec to see who they recommended for my car (EFI with datalogging sensors throughout), and I was told the details of three different companies. In ballpark figures, one quoted $2500, one $4000, and one $7000-!!!
It pays to shop around!