I've noticed that they carry over from campaign to campaign.
That is, I've gone from single mission mode to a campaign and the single mission purchase file carries over until I got through a few missions.
I've also gone from the MW2 campaign where there's no mech purchase option (greyed out) to solo mission and then to a new campaign while still having the mech purchase greyed out.
All of this is resolved reliably only through a restart of the program.
Also, occasionally the variants listed in the purchase screen appear to be random from mission to mission.
Any answers for why this happens?