I have owned 2 of the Jonathan Browning Mountain rifles, both were bought used, both were in .50, and both were just the rifles. They were NOT the full set or commemorative versions. Rifle #1 was a brass mounted version that was missing a barrel key. I paid $325, modified and replaced the key, and shot it, but really wanted the iron mounted version. The single set triggers were great.
After about 3 years, I found a mint condition iron mounted rifle in .50 at a PA gun show and paid $500. It had much nicer wood than the brass version. I sold the brass mounted gun a month later for $500, which I was very happy to get (a right buyer at the right time, as he felt he got it cheap and I was happy to not lose money, a win/win in my book). Both were/are great shooters.
Mine both had decent springs. I recently saw a boxed set (without the commemorative plaque) for sale at a show for $850. If it was the .54 I would have bought it, but passed.