it depends on the parts. Newer parts have much less gold. Older parts (especially those from the 70's and 80's) have more gold in them. You can google for companies that do the reclamation and get some basic prices for things like motherboards, CPU's, memory, and other parts.
The biggest problem with computer recycling is that it is hard to get your hands on just the valuable computer parts without also having to take in a bunch of hard to recycle plastic and junk. Nobody is going to tear apart their old mainframe and hand you the good stuff. They are going to expect you to take the whole thing. At that point the amount of gold you may recover might be significantly less than what it costs you to dispose of the unwanted/useless parts.
Again, google for it. There is plenty of information out there already. There is also a scrap metals forum on this site where it gets discussed quite often.