For pricing, Purdey is Brand Value level 1 (BV1) and the gun in question was an Orignial Quality grade 1 (OQ1). Current Condition is the other factor for a ball park price. Doubt the gun is pristind (CC1). So, CC2 is slight use, CC3 is significent use, and CC4 is heavy use but no abuse. BV1-OQ1-CC2 = $28,000, CC3 = $21,000, and CC4 = $16,000. Multiply by a factor of 1.5 to 2 for scarcity and desirability of 20 gauge. That makes you a range of $24,000 (1.5 X $16K) to $56,000 (2 X $28K). If at all a righteous gun, it is approaching a bargain. If you have questions about this market model system, ask --- oh, yes, expect some wiseacker comments as there is a gunning runbattle over it.
Give a hint what part of the country you are in and someone will recommend a qualified gunmaker to look at it if you decide to buy. If you decide not to buy and your friend is going to sell, I'd be interested in a chance at it.
