I was figuring a buck + for the axle, I figured that it would cost you 4 - 5 for the materials to get the tube frame plus the welding time. The tires/wheels will be another buck. The electricals would run you close to a buck by time you add it all up.
The small big tex type trailers can be had for <$8.
It sounds like you have most of the goodies to do this. I look forward to seeing how this looks.