I sold three 21's in the last two years. All would have exceeded what I understand your desired buying price. Some gun names and models bring prices that exceed shooter prices and tend to be in collector price range regardless of condition. Browning superposed, Winchester models 12, 21, 42 and 70's bring top dollar both because they are sought after by collectors and shooters both who have money to spend.
On top of that 21's have been a custom shop gun for fifty years so the total numbers are modest. Take away the ones never shot and those used to be upgraded and your purchase pool is getting smaller. The flip side is that if you buy one and take good care of it it will never wear out and when sold would bring a modest return on investment. The ones I sold netted me about eight grand after 20 years
