Based on recommendations on this site (search archives) I bought the Premium Harbormaster 20-22 foot boat cover from Walmart...ordered through
www.Walmart.com and picked it up at the store in thier site to store program (free shipping). Not all stores carry the cover so I would just order and pick it up when it comes in...if you are not happy you can return it to the store.
For $100 the cover is very good quality...I have a 20 foot center console...I am going to build a pvc frame to support the cover to keep water from pooling on top in a few places.
I purchased a $400 custom cover years ago for another boat and this cover appears to be made from very similar material.