So not long ago I decided to build one. I've build this unicycle with the help of a friend and it was a great learning experience.
This instructable will just briefly show how the unicyle was build and proof that you don't have to spend $150,- on a 28 inch unicycle.
The unicycle is made out of an old (modified) unicycle frame, an old bicycle wheel, a spare unicycle hub and a saddle of course.
We took an old bicycle wheel and took out all the spokes, then we put the spokes in the unicycle hub and started reassembling the wheel.
For the frame we used an old unicycle frame. The frame needed to be extended in order to fit the 28 inch wheel. To extend the frame I cut off the arms, extended the frame with bicycle tubing and welded it together again.
The unicycle cycled great until the axle started slipping in the unicycle hub. The unicycle hub was pres fitted together and for some reason it gave up on me so I decided to weld the hub together.
This project is reasonably easy for anyone with a bit of welding experience and will save you some money if you have all the parts.