Although other possibilities exist, there are two common problems that caus
e your scenario on your vehicle:
1. The screen in the charcoal canister has rusted away and the loose charc
oal is blocking the vapor vent path.
2. The canister filter is clogged, similarly preventing venting.
In either case, replacement of the offending component is recommended. The
canister is by far the more expensive, so if I find a canister problem, I
typically recommend spending a few extra bucks to replace the filter as wel
l while everything is off the vehicle and apart.
The estimate sounds a bit high unless they're including a significant amoun
t of diagnostic charges (which may indeed be reasonable for finding the loc
ation of the issue). For parts and labor to replace the canister without a
ny diagnostic time, I'd expect something in the $300 to $550 range.
Maybe you should shop around.