Even Costco canvas printing for wraps is better than $15/sf for their "generic" printing. Check such places as West Coast Imaging and Simply Canvas for comparison pricing of what you would pay if you farmed it out. $20 is usually a minimum for Canvas production and it goes up from there, up to $60 and more per square foot for smaller prints and gallery wrapping on a stretcher. Most that I know who print commercially charge by the square inch and then add the cost of finishing, including coating and stretching.
I charge by the square foot and won't deliver without coating and most want a wrap or stretched on a frame, which adds to the cost/square foot.
If your client is only offering $6.50 sf, I'd say walk. Canvas is pricy, comes only in 40 foot rolls (it's a manufacturing QC issue, I'm told), takes more effort to print, handle, cut, coat, etc. You print on one day, coat the next and on the third day you mount or stretch. It takes time to do it right. Even at wholesale rates, most will charge you 3 to 5 times that. If this will be a large job with thousands of prints at your leisure, then it may break even for you, but I'd not do it for that. I don't even think Costco with their 9 different sizes offers it that cheaply.
If you are a better than average printer, you should even charge more. Their offer is pretty much bottom-of-the-barrel for even the least expensive matt papers, IMO.
Run, don't walk. away! If you want to help them out, refer them to any of the on-line venders where they will get a reality check and perhaps offer you a more realistic rate for your efforts.
At $6.50 per foot, not counting cleanings, rejects, glitches, etc. you'd be doing well simply to cover your basic materials and supplies, not including wear-and-tear on both you and your equipment. You are right to be scared!