Quoted from davewtf:
Go to Boston pinball or buy a price guide
Even with these price references (I use pinside [derived from boston/ebay], boston pin/ebay data, and guide) I've seen the price police blast sellers asking what seemed in-line with these prices.
I generally take all of these as a reference, make my own adjustments, and then just go plus or minus from there based on condition, desirable mods (I don't like mods much, so generally don't pay much for them), etc.
If you're just building up your own collection, ultimately just use the guides as a ball-park reference, and then pay what you feel its worth. If you are looking to flip pins, you have to have a good deal of experience with pricing and market.