Quoted from Schwaggs:

You need better pictures of the playfield, boards and backglass to know for sure. These games were played A LOT and many of them are rough.
Look for ball tracks through the black paint around the arch at the top pf the PF
Raised, lowered, loose inserts.
Wear to the wood around the inserts and slingshots.
Cupped inserts (so much that the trajectory of the ball is impacted or the ball can get stuck in the F I R E inserts at the top of the PF).
Faded color and cloudy inserts.
Battery damage (corrosion) on the CPU board.
Burned resistors on the driver board.
Wimpy flipper power.
Look at the backglass with the machine on and look for flakey and missing paint. Really easy to see this from the back of the backglass (original reproductions NOT available so don't expect to find this until CPR makes one)
Take the PF glass off and try all the switches to make sure they register, put balls in the locks to make sure the multiball works.
Common that the playfield plastics are warped but they all should be there along with pop bumper caps with Firepower written on them (used plastics come up for sale frequently so don't freak out about this)
If the machine works and has several of the defects above, you are at $500. If it doesn't work, less.
If the playfield is relatively level and not worn too badly (hard to find one without wear) - 750 - 1000