I'm looking at doing it now. Price depends on speed. eBay seems to be the best source. It is a bit complex but not over the top as it looks like a two hour job. Check out http://forums.macrumors.com/forumdisplay.php?f=1 for those that have done it.
Different options and some prices that I have found.
• 2.00GHz x4 - E5335 - 80watt - SLAC7 or SL9YK or SLAEK (G0)
• 2.00GHz x4 - L5335 - 50watt - SLAEN (G0)
• 2.33GHz x4 - E5345 - 80watt - SLAC5 or SL9YL or SLAEJ (G0) $120 x2
• 2.66GHZ x4 - X5355 - 120watt - SLAC4 or SL9YM or SLAEG (G0) $200 x2
• 3.00GHZ x4 - X5365 - 150watt - SLAC3 or SLAED (G0) $350 x2
I think this is not nearly as simple as upgrading memory, but you can learn from other's mistakes.
The first time the boss at Anandtech tried this upgrade, he trashed his processor board. Later, he did the upgrade you are seeking and described it here:
There are sites and long threads here and elsewhere like MacRumors: Mac Pro devoted to the subject and a two part Youtube series (there is another guy with 10 clips but not as well done and rambles quite a bit).
eBay is best bet, for what Amazon has you could buy a new Mac probably.
And you do want to check and see what others have said and done, again the photos, specs, what to look for, and video is worth 1000 words.