The logic board itself is probably between $300 and $550 depending on your model and where you buy it (eBay, an online store, or from Apple). I thought about buying one for mine and replacing it myself, but figured that by the time I spend $400 - $500 on a logic board, I'm halfway to a refurb MacBook and just a little more gets a MacBookPro. That doesn't even include labor to install it. The PB itself (when both slots were working) was worth about $750-$900, so another thing I considered was putting 2/3 of the value back into it to fix this issue. (Kind of like paying more than a car is worth to get it fixed.)
On top of that, I read several reports of the replacement logic boards failing in the same way (after 6 months or a year) -- that made putting $400+ into repair even less worth it.
Now if you have AppleCare still active, you should be able to get it fixed at no cost.
So I saved up some cash and bought a refurb MBP. I had hoped to keep the PB G4 as my main machine for a couple more years, but it wasn't to be. It will still get use around the house.
Here's one place to get a logic board:
http://www.ifixit.com -- I've ordered other stuff from them and they were great.
They also have excellent guides to show you how to replace just about anything, including logic boards. I've read here about users doing logic board self-installs and being successful.
Good luck!