Thompson parts' prices seem to be on the rise sharply, these days, and while it's been a while since I bought any spares for mine, I'd imagine somewhere in the $100 range for a stripped M1A1 bolt (by "stripped" I mean minus the extractor...there isn't anything else on the bolt.) IIRC, IMA had complete M1 bolts for about $100..not the same as what you may be looking for, but completely interchangeable.
I happen to have a Savage WW2 marked bolt that was an extra for my M1a1 class 111 Thompson before I sold it. It's in nice condition and I could part with it for a for a lot less than $100.00 for a fellow gun boards member. My bolt is complete with extractor and I have other spares too.