Answer:
Back then not much, now could be worth up to $45.000 dollars if it is is really good shape and has the right accessories. A convertible could be worth much more than that.
It also depends on the engine, which was a base 383, then the 440, then the 426 Hemi. The Hemi changes everything if it's original to the car making it worth exponentially more money. If you have one of the very rare Hemi convertibles, then you are looking at a million or more.
Average price of a show quality RR is about 35 to 45K for base models and up from there as you start adding desirable options like 4 speeds, bigger engines, and other packages like the A-12 440 six packs, Hemis and ragtops.