I don't know the quality or markup on the aftermarket part, nor the going
labor rate at the Firestone in your area, so it's difficult to say whether
it's high for what you're getting. If its the lowest price you're able to
find, then it's probably a pretty good price.
As to why the dealer wouldn't give you an estimate, I don't know. It's
pretty simple, really. Price of evaporator plus two pounds R-134a plus
labor to replace evaporator.
I ran a rough estimate for what we would charge here, and it's about $1200
for the evaporator and case assembly, or about $1150 for just the
evaporator core itself. Unless you're near a high-priced major
metropolitan area, dealer prices in your area will probably be lower.