place I went to get a quote charged me $99 just to come out to look at my place. Then they were going to charge me $279 to replace my existing outlet
This is in Southern California, but I think I could have gone cheaper with another vendor.
I decided to get these portable GFCI units instead from Home Depot since if I move, I can take them with me.
I heard if your building is not up to code (say it's an "older house" that it may be more than just replacing the plug.) from the electrician that quoted me for the job, it might cost more.
I spent my money better by getting the various devices for home and work (since my building was up to code anyway):
I use these at work and place a surge protector after it.
At home, I use these. One for each outlet:
I got these so I could put GFCI on both plugs since I run several tanks at home.
You can also get this one if you have 1 aquarium system:
I didn't like this because since i run 4 tanks at home, I could only have GFCI protection on the one outlet.
These can be found at Home Depot and it may not be near their "surge protector" section all the time (as I found out).
The company's name is TCRI at www.tcri.net
I think another place I heard charged $75 for a GFCI installation. Much better than I was quoted. I like these portable units for my application better.