I have an inline 20A GFCI for sale (link below). These are great if you need to share an outlet with equipment that doesn't play well with GFCI (e.g. electric dryers) or if you want the GFCI in an extension cord. I've run it up to about 25A (6 kW) without any issues, but wouldn't go beyond that. The unit works fine, I just decided to upgrade to a 50A breaker so I can run my BK and HLT at the same time.
I'm thinking $30.
http://www.mcmaster.com/#7228k13/=d8ie4s