That is not a pocket navy. First Pocket Navys are not 1851 Navys, 2. Pocket Navys are .36 caliber and not .31. The pocket navy was built on the .31 1849 frame with a rebated cylinder (like the '60 Army) and a .36 barrel. What you have posted a picture of is an 1849 .31 caliber pistol. These are quite common used and not a premium item. I have 2 .31s marked Replica Arms, they were made by Army San Marco and are nothing special but are serviceable. As I remember I gave a friend $85 for one years ago and the other one was maybe $125 2 or 3 years ago but both are in extremely good condition. If it was very good and functions properly I wouldn't go over $125 but that's just me. Actually the reason I got the 2nd. ($125) one was it's a Wells Fargo (no loading lever) with a 6" barrel which is quite rare.
Just sold one (ASM .31 cal - ) for $150 - one issue - it was unfired but had been dry fired repeatedly resulting in nipple damage. I found the threads on the early ASM guns were M5.5x1 and all the replicas now are M5.5x.9 - there were no nipples anywhere that would fit that gun.