Classic Accessories Storm Pro and Dallas Manufacturing HarborMaster Premium are some decent quailty covers for cheap. 600 denier poly and decent stitch work.
They describe them as trailer-able and have 5 year warrantys. I remove mine for trailering so I can't attest to that, but its held out weather and dust for 5 years for under a hundred bucks. Use water pipe insulation to prevent tearing. If it tears, send it back I guess.
Check online for best pricing. WalMart is currently carrying the HarborMaster brand. Premium is much better than Silver line. Classic Accessories is a little nicer, but still under $200 for the 22' version
http://www.mytarp.com/stormpro-boat-cover.aspx