However, it may be the rarest of all the first-gen mustang convertibles (71s, that is). Think about it this way - they made over 40k 70 mach 1s, and I think over 70k '69s.... 71 convertibles are jewels.
as already mentioned, 1971 was the lowest production year for mustang convertibles, and rare is relative. i seem to recall that 1970 and 1972 were only marginally higher in production numbers. however, i haven't noticed that the scarcity has manifested into higher prices. i have noticed that '73 convertibles seem to be priced a little higher, and they made about 14,000 of them. it probably has to do with '73 being the last year of the first gen convertibles. they also made gobs of '65 convertibles, and they seem to be the highest priced of mustang convertibles. the principles of supply and demand dictate the market. at this point, there are more buyers looking for '65-'66 convertibles than there are '71's. i have one of the rarest mustang convertibles of all, a '71 with a 4-speed (1 of 183). i get lots of nice compliments, but no one is beating my door down to buy it. my advice would be to not pay a premium simply because of the production figures. good luck.
When you use the "R" word it justifies an outrageous price request. Seriously, I have been seeing quite a few 71 verts going on the auction site and not selling at $8-10k. Just check the car over real well for rust and damage.