KX80 Rebuild

the most important thing would be getting good information. Get a shop manual for the bike before you even think about a project like this. Next, make sure you have the tools for the job. Of course, you can always improvise, but try plan ahead.
Get the manual, look at where they suggest using special tools and go from there. Most of the time, you can get away without the tool, but sometimes it's a must. A gear puller for the magneto, and a splitter for the cases is a good place to start. You may need to use an impact wrench to loosen the clutch hub. Snap-ring pliers always make life easier. And a micrometer is a must if you want to "do it right."
As far as parts go, the cost will depend on what you're planning to replace. If the crank is bad, don't bother trying to fix or balance it yourself, either send it to a shop to get balanced or buy a new one from Wiseco (OEMs are expensive!). Of course, replace all of the gaskets, seals and O-rings w/ new ones. If you find any shift forks or gears out of specs, might as well replace them while you're at it.
All in all, I bet that you could do it yourself somewhere in the range of $250 - $500 bucks (depending on what you have to replace).