Hum, I think a free swivel wheel would only work for very cheap designs. If you're not convinced, you can search for 3 wheels vehicles on google image [1]: none of the vehicles there have the 2 wheels + 1 free rotating wheel design that you recommend.
I think that's more of a speed thing than a cost thing. At higher speeds you want to control all the wheels to ensure you don't go sliding off into a utility pole or something.