Sounds like you've got a winner there! Yeah, the pelts are worthless now. $10.00 ea seems a bit optomistic to me from looking at the fur auctions last year....
$5.00 ea is way more than fair. He doesn't have to skin, flesh and dry them. I'd tell him to get 'em out asap, and come back during trapping season to catch any that might see the vacancy sign. I'd give him the go ahead to take care of some 'coon as well (this Fall), they do a pretty good job on nesting waterfowl.
Locally, the 'rats don't get prime (have good fur for the garment industry) until late November. They are about 50% to 70% prime the 15th of Nov, when trapping season opens, but it also depends on how cold it is in late Oct/early Nov. they're called "blue backs" then because you can see where the fur isn't prime on the inside of the skin once they're skinned. Once they are fully prime, the bluish color isn't there. Where it's legal to trap into the Spring, the 'rats fight during breeding season and the pelts are devalued because of the cuts on their backs.
Also, make sure you have it figured out where the dead 'rats will be put. On your place or will he take them with him and dispose of them? That shouldn't be a problem during trapping season because he'll take them with him to skin them out, but he doesn't have to skin them now.....