If you;ve got to buy it and its for a motorbike nothing it seems comes cheap. To go cheap you need to think outside the norm. If your lucky you will have a lathe and can make something. Most of us don't so its down to finding something that maybe has been made for another purpose. Something made with cable ties tends to be cheap.
Alternatively this is the cheapest bought option I have seen.
http://www.google.co.uk/products/ca...=X&ei=saYYT_iUDMen8gP5qrWiCw&ved=0CIABEPMCMAI
If you do come up with something novel please post it on here.
I for one can get a nice 5 inch Satnav for £50 that is far superior to the Zumo and Rider but its not waterproof and doesn't have bluetooth sound. The lack of which are major drawbacks on a bike.