Buying an aquarium

Hello, my name is Evan. I would like to know what a reasonable price
for a 20 gallon sal****er tank is, I am also interested in what you
think a 20 gallon freshwater would cost. My lfs is offering $170 for a
20 gallon freshwater tank (includes: stand, tank, filter, gravel,
heater, thermometer and air pumg). I am also wondering as to what you
think a reasonable price for a 55 gallon sal****er tank would cost.
How much do you think a freshwater would cost? My lfs has offered $500
(includes: tank, stand, filter, heater, gravel and protein skimmer).
Thanks for any input!

Evan