JBMR .50 Cal.
They were discontinued in the early 1980s. They were manufactured in .45, .50, and .54 Calibers. They came with brass or iron furniture. The iron (butt-plate, toe-piece, trigger guard) furniture models had a pewter-tip forend cap.
I've seen mint condition .50 cal. with brass selling for $400 to $550, and mint (with box & papers) iron ones going for up to $800. Browning made some limited editions JBMR that now sell for well over $1.5K.
If your rifle is well cared for and is the Brass furniture model, $500 would sound about right - - if a Iron-model add $50 to $100. BTW - - 2-years ago Cabelas Gun Library listed two JBMRs both with the iron furniture for $750 each . . . they sold almost immediately.
Hope this helps.
I own 6 of these fine rifles . . . all Iron models. Three in .50 cal & three in .54. They are outstanding for a "production" rifle. I hunt extensively with these rifles. From deer to bear. They are VERY accurate. AND NOPE! Don't want to sell any of these sweet-shooters.