RepairPal Estimator is a website / tool that allows you to input the make of car, model, year you have, your zip code, and what type of repair your car requires. Then it will give you a range estimate for that that type of repair often costs, and a map to the location of local mechanic shops. The service includes scheduled maintenance visits as well as repairs.
It estimates both what it will cost to take your car to the Dealer, and what an independent mechanic might charge. The estimate is given as a range because different shops will charge slightly different amounts for the same repair based on their shop labor rate and the materials used.
Because the data depends on inputs for the repair costs, the database currently covers from 1990 through 2008 or so. If your vehicle is outside of those years you would have to pick a vehicle within those years to get an estimate, with associated decrease in accuracy.
I found the Repair Pal Estimator to be a useful site for a first estimate of repair cost.