A round ball barrel in .50 caliber for a Thompson Center. What model?
Green Mountain used to offer drop ins and these are likely to most common. They haven't offered them for a number of years as I understand it. I've seen folks mention Rice but I saw nothing in stock last I looked. The T/C custom shop, FOX River I believe, used to sell round ball barrels. If you find a good used T/C Pennsylvania Hunter they came from the factory with a round ball barrel.
Price, I just sold a mint Green Mountain LRH, not for round ball, for $300. I could have asked $400 but I sold it to a friend. Condition is everything, a really good one easily $300 - $400 would be my bet. I also have a Green Mountain round ball barrel I would not sell.
If you can live with a single trigger model the PA Hunter, complete rifle, might be a cost effective option. I've got a PA Hunter Carbine and love it.