The cheapest fighter jet you can get is the L-39 Albatross from the 1960's which will run you between $200k-1mil depending on work required for flight worthines

The cheapest fighter jet you can get is the L-39 Albatross from the 1960's which will run you between $200k-1mil depending on work required for flight worthiness while on the higher end a supersonic trainer like the F5 would run at least $2mil. Considering how rare the parts are and the difficulty of finding someone qualified to actually do the work, $4mil is certainly very reasonable, especially if the Spitfire is actually flight worthy.