Here’s a detailed Instructable on building a single-wheeled bike trailer, very similar to the $300+, critically-acclaimed B.O.B. trailer systems, for something like $20. Aside from the coolness factor, one vs. two wheels obviously gives you a nice lowered rolling resistance.

My favorite quote from the build is the description of his hitch system (step 8). The Maker bought the B.O.B. hitch, came up with a better version, and returned the commercially-produced one back to the store!