The longer barreled guns are not really a good metric to gauge pricing trends. The longer barreled guns have always been less desirable than shorter barreled guns of the same make.

Snubs worth more than 4" guns, 4" guns worth more than 6" guns and so on. 850 is a good price for any pre lock 686 though, with Guns.com having one listed in box for 1400 bucks.