Most companies aren't as bad as people think, just put yourself in their shoes. They want to pay you out but need a paper trail, try
http://www.vmrcanada.com/ and see what comes up. The only thing I find unfair with most companies is how quickly they want you out of the rental when negotiating even when you have the coverage.
Also you can hire your own vehicle appraiser. Appraisers are incredibly busy and don't always have the time to investigate as much as they would like, I'm not sure the reasoning they didn't accept your comparables but I'm curious. Most auto adjusters are new and don't really know much more than you, it's a high turn around position.
On the other end of the spectrum it is a vehicle that loses value fast, not every dollar you put into it will or should be accounted for. What kind of vehicle is it and how many km's etc?