I would have thought that the purchase DC should relate to the wealth bonus of the bribe target...
I'd say that bribes would act as a circumstance bonus to a diplomacy or bluff roll (and conversely, someones current wealth DC would act as a bonus to their sense motive (? seems the most appropriate thing))
ie -
Bob wants to get backstage at a concert, and Frank is tasked with stopping him. Bob tries to convince Frank that it's ok to let him in, and rolls a bluff roll (attempting to get someone to act in a certain way for a short period of time), versus Frank's sense motive (adjusted by the table for resisting a bluff based on extenuating circumstances) . He gets a total of 10, versus Frank's 15.
He decides to sweeten the deal with cash. As soon as this happens, Frank's roll is immediately increased by Frank's wealth bonus (because if he's rolling in cash, he'd be harder to bribe). Bob picks a purchase DC for the bribe, and adds that to his own roll, and the results are checked again.
In this case, Frank is an underpaid roadie, he's at a wealth bonus of 3. Bob decided to go with a purchase DC of 10, just to be on the safe side. Bob's roll goes to 20, Franks ends up at 18, so the bribe was successful. Bob adjusts his wealth rating by the given amount, Frank increases his wealth rating as if he'd just sold that money (-3 to the purchase DC, gain as if you'd had to buy it, so his wealth goes up by a point) and he lets Bob through.
I'd suggest that whether a specific character will take a bribe would be up to the GM, as would the effect of 'haggling' with bribes (ie - the initial bribe wasn't enough - can the player try again, or is the target insulted? Will the target tell the player how much it will cost, etc).
