Just my 2 bobs worth as I am reasonably new to the vmx scene.
I decided I would look for a bike to restore as my first vmx bike, $800 Gumtree Suzuki RM370 turned into $6000 and it wasn't a 100% restore . That was 2yrs ago when the dollar was good ! When you discover the frame was bent, forks needed chrome, new spokes ( I used the original rims) paint the tank, powder coating, new nuts and bolts, replating, good shocks, decent ignition, new plastics, new carb, new pipe and muffler, complete engine rebuild etc. It adds up on a restore and I did 90% myself.
On the other hand I paid $3000 for my 76 RM250 with Fox shocks and a brand new spare exhaust, had been sitting in someone's shed for 20+yrs. Didn't touch the engine, set of shocks, fork seals and raced it for a season and a half with no problems at all.
Then there is the 74 YZ250A i just bought, I paid more for it than the RM370 cost me to restore.
It is reasonably original, there getting rare, has history and i WANTED it so I could race pre75. So I paid good money for it, I'm not banking on them going up in price to fund my retirement I just want to have some fun riding them and there getting harder to get these days.
As I tell my friends that ask me, if you really want one of these old klunkers pay a price YOU are happy with and go have some fun !
Cheers