Most photographers that I have seen will charge anywhere from two to four times the cost of prints. If your client wants a standard size canvas gallery wrap, they will be looking at paying around $300, if not more. I think about what I would be willing to pay for prints and things like gallery canvas, knowing the quality of work I’m getting from my photographer and the print lab. Do some comparison shopping within your area and see the pricing from your lab. Some photographers will include the price of a canvas in their package or session fees and “gift” them to their client. That way it’s a surprise but at least the base price is covered.
I’ve never had to flesh out pricing though so I’m sure someone else can give you a better idea.
As for watermarking, I would definitely put something non-intrusive on the image that just shows it’s yours. They can still be cropped out of cloned out by some dastardly villain, but it’s a little bit of a deterrent and it helps with things like pinterest/facebook sharing.