It may not cost anything to repair. You won't know until the lock is removed to see what's going on. It may even work just fine when taken off of the stock, indicating it's binding in the mortise or perhaps even tightened down too much or improperly installed with the sear bar pushing against the trigger. If it doesn't work correctly outside of the stock there are a number of things to check; the internals may be gummed up with dried oil and just need a good cleaning, the screws holding the bridle could be loose (a common problem with TC locks), the sear spring could be missing or stuck in a compressed state, the tip of the sear could be broken, or both the half-cock and full cock notches on the tumbler could be damaged (seems unlikely that both would be damaged).
Anyway, if the sear spring is missing or there's damage to the sear or tumbler it's likely Thompson Center will send you a new one. About six months ago I dropped the sear spring from one of my Renegade locks in the grass under the shooting bench on our rifle range, and could not find it. I emailed TC asking how to order a new one and they sent one gratis in just about a week.
I would not let a non-functioning lock on a Renegade stop me from buying a gun that's otherwise in good shape. Heck, it's a bargaining chip...….."Aw man, the lock on this thing is broken, and getting it fixed or replacing it could be pretty expensive. Tell you what, I'll take a chance that I can fix it if you'll let it go for $150." If bargaining doesn't work, it's still a good buy at $200.