I picked up my project SI with beautiful playfield and backglass for 250.00 last year, but it was a dirty mess and dead.
After dropping about 650.00 into it for AllTek boards, plastic set, power supply rebuild, connectors, LED's, lights and rubber kit, cleaning, waxing, etc... I'd have been money ahead buying a working one if I could find it but I really enjoyed bringing it back to life.
I'd say for a nice one with no real playfield wear 800.00 to 1000.00.
Good luck! If it has the original MPU in it, be sure to remove the leaking battery ASAP!
Tom