I did some additional checking, RailEurope is selling 5 day roundtrips from LGW to Victoria station for $20, and STN to Liverpool station also for $20. Assuming it's just the Tube between those two stations, maybe $45-$50 total for the round trips.
Don't know if discounted fares like this are sold in London though, or would I have to order them before I left the States?

Jeff