Nice build mate, coming along nicely, $900 for canvas is about right unfortunately but this all depends on what material you are wanting to use, I was lucky and got my canvas work done for $650 but this is only because the guy really wanted to do it to get his new business out there and as it was a custom out of the norm job, apparently material alone was around the $550 mark. I made my bar work so the tent sat lower like yours although not as low as you can see, the RTT just clears my head when set up.
I used a heavy duty canvas as it will still "breath" and not keep moisture inside, I also added a pocket in the rear for rubbish, wet clothes or dirty recovery gear. 1 thing I would like to have added would be some extra storage pockets up at the headboard on the inside, would have been handy and I guess I can always get this added in later.
Have you given any thoughts on having doors on the canopy for security reasons?
Cheers