Like CoffeeQueen, I decide based on the amount of dispensers. I'll do a four-pump blue station in good condition for base pay if its on route, or near my route. Otherwise, I'm aiming for $20-$25, or more, depending in different factors.
There are too many factors to just state a price.
Is it on your route already? How many pumps? Time of day restrictions? Is the station typically well maintained (less infraction photos)? Any reimbursement for gas/ store purchase? Picture of every pump or just one? Is your area highly saturated with shoppers?
Consider all these things when deciding your price.