What kind of cover? Winter cover? Cover while it sits in your yard? A custom made cover could cost you $1000-2000. Most canvas shops will make one for you. If you are looking for a cheap option sew a couple of car covers together.
My boat came with a Fairclough boom tent (heavy winter cover) that goes down to the toerail and it was over $1400. On the plus side, it is likely a good time to buy one.
Bob