You might consider that there are a number of different Sunbrella fabrics in the Marine line. Marine Sunbrella is not "Waterproof"--but breaths better. Sunbrella plus is more water proof--enough for most applications. Sunbrella Supreme is also waterproof, and has a flocked backing. Sunbrella SeaMark has a vinly backing which makes it the most waterproof of the Sunbrellas, but less breathable. The premium fabrics may cost more. I have used Sunbrella plus, and find it is a good compromise.
I think that the $2500 to $3500 is a good number, for a quality camper back. But there are lots of options. A good canvas person will meet with you, go over the boat and make suggestions. See what they want to do, and if it makes sense; not just the cheapest job. look at the quality of the clear vinly, if they use Tendra (Gortex) stitching which will last for the life of the fabric, or dacron thread which will have to be resewn after a few years etc...Where are the zippers located? Are there going to be screens plus windows, or just windows? Look at ventiallation in the summer (for example on my Top, I can remove the first pannel for air flow, but the frame is still free standing.