Oh, there are tons of things to be done with this. First off, the PCs are likely to want to find out whether anybody has claimed the manor in the time since the deed was first lost - at which point the PCs can decide whether they want to fight for their property rights or win friends by surrendering the deed. Unless the property ends up vacant and they sell it off, they end up with a home base (whether owned, or allowed to stay for their good deed) right next to Absalom. There's lots of advice available for city-based campaigns and it's a huge subject - the CRB's notes on urban adventures can be of help, as can Ultimate Campaign. But the short version is that by giving the players emotional ties to a particular campaign location, you open a lot of possibilities involving recurring NPCs, local politics and other things that the usual group of vagabond heroes don't usually concern themselves with. It's best of all for character-driven, story-heavy campaigns; but even a kick-in-the-door combat campaign can find uses for it, because NPCs with problems that need killing know where to come to ask for help.
Suitably old property may itself be an adventure site, too. Hauntings or infestations; old smuggling or cult compounds beneath; a door in some out-of-the-way room that opens into someplace that is definitely not in the house... You get the idea.
As for its resale value, Ultimate Campaign informs me that a standard 'mansion' - fully furnished - costs 5160 gp to construct. A party eager to liquidate an older and largely-unfurnished version could readily expect - my estimate - 3500 or so, but by taking time to replace furnishings and find the highest bidder, a manor so close to such a prosperous city could probably fetch double that.
