Changing 2 prong outlet to 3 prong?

Without scaring you about the NEC (National Electrical Code) requirements, I'll talk in layman's terms. If you have (and you probably do already) an equipment grounding conductor (bare copper or green wire) in the outlet box, all an electrician has to do is change the receptacle from a 2 prong outlet to a 3 prong outlet ... that's it.

My guess is it could cost close to $40.00 per outlet for the conversion only.

Most of the cost will be labor. Sorry it is just too hard to estimate the labor cost in your market, or how difficult to add another wire to the outlet box.

The NEC says to replace with a grounding receptacle when no equipment grounding conductor is present you need to run a wire back to the "Grounding Electrode System" (which is the electrical panel, the ground rod, the first 10 feet of water pipe where it enters the building, or a wire that connects the previous parts). So how long it will take depends on how hard it is to fish wire into the walls from your attic or crawlspace and then routing it to the grounding electrode system..

There is another option allowed by the code that may be cheaper. You can replace the outlets with GFCI's without extending a ground wire and label it "no equipment grounding conductor". It stops current if some current leaks, so it does provide some electrical safety preventing shock, it doesn't provide an actual ground, but it does allow you to conveniently insert a 3 prong cord.

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Just put in some GFCIs for three-prongs. They don't require any grounding to work properly, as authorized by the NEC. If you don't actually need grounding (e.g., you might for some types of audio equipment), then don't bother rewiring the house for grounding a few outlets.

Change 2 Prong Outlet To 3

$250 plus the cost to open and fix the wall.