The workshop screen days that a mig costs $250,000, but if I have exactly that much, and try to build one, then when I return to the geoscape I immediately get a message that my mig production is delayed for financial reasons.
Then, when I make more money, it goes ahead. I am not sure how much money it took.
I think I built a second base with a radar, store room, living quarters and hangar, which left me with exactly $250,000. That's if you want to try and recreate this bug. The two bases were in europe and north america, if that affects the cost.
Edit: I probably built an additional hangar at my europe base too, to house the mig.