I own 7 canvas ones and have had no issues with them getting dirty cuz I am real careful with them. One time I did get gum on one of my evening bags from the theater seat (darn dirty theater!) and was amazed how easy it came off, good as new.
Yes, I'm careful with her, she doesn't get to touch the floors, or tables at food places, OR theatre seats...but really, no problems with her, had her for almost a yr now and still no scratches, no marks, no scuffing...
I only have the sukey boston in canvas and have no issues with it easily getting dirty part and sometimes I would bring it with me to work - I work in an elementary school where dirt and germs are rampant.
i wouldn't think its worth it to buy a gucci canvas for $900.. but i depends on people's personal choice.. a manager at a gucci store one told me that he wouldn't prefer canvas over leather, either.. just my opinion tough!