IMHO the dent doesn't compromise the frame's integrity.
In good condition a bike like that, a PSN-10, would go for somewhere in the $300-400 range. The dent alone knocks that down by 50%, other condition issues knock it down even more.
Personally, if it was my size, I wouldn't pay more than $100.
Yes and no about the frame set's structural integrity. The top tube is much much weaker, with the dent, but the tube will not fail due to riding the bike. That dent, however, destroys the monetary value of the frame set completely in most people's minds. That said...
$100 tops, but $50 is all I would pay. The value there, now lies in the component group only. My opinion, of course.