I've been through a couple from eBay. The first one was nylon, like a wind breaker, and didn't last a year. I think I got it for $30 bucks. The second one I have had for about a year, it's canvas and it is showing it's sun wear. I got that one for about $60 bucks.
Next time I will buy a quality one that will last. Sorry I can't recomend anyone, just passing on my expieriences.
After going throught he blue tarp syndrome with previous boats, and trying to find a pre made cover from all the usuall marine supply stores, I have come to the conclusion that there are not any "good" cheap covers. The expense of having a good canvas cover made to fit the boat was worth it. It has lasted 3 years so far, and is still in goood shape. It has paid for itself by protecting the boat from all the effects that weather, including UV from the Sun can cause. Jusy my opinion. Mark/Time Out ch 72
Look at the semi custom ones at Cabelas.com. You should be able to get one made out of very thick, water repellent cotton canvas that has strong loops for securing it, stronger than grommets. About 99.00. I had one on a 19 foot runabout that lasted over three years in SoCal sun. It comes with nylon rope but I generally use just bungees.....quicker and it still creates a "seal" at the gunwales.
Consider getting one of these and then covering it with the standard grey poly tarp you can pick up cheap at Home Depot or Wally's. Let the cheap poly absorb the UVs and replace it every two years or so.
Avoid the covers for trailering.....they are much more expensive.
One other thing you might try, K-marts and some Walmarts may carry the Stearns brand of universal fit covers make out of the same material as the Cabelas. Call around and see what they run.