I just rehabbed a 88 MOTO 4 200 that I picked up for $300 off of Market Place. I got a fully operational machine on the trails for under $600. Parts are cheap and plentiful on Amazon.
All I needed was a coil, a carb (the price of a carb is not much more than a rebuild kit, saving a lot of tedious work), a new petcock (the handle was broke off) brake pads and front brake cables).
Oh yeah and redneck engineer the the air box as the door and cage were gone and I was unable to source them for a reasonable price.
Good luck on your project and have fun.