Now if your buying a whole aquarium setup with tank, hood, stand, heater, filter and more then I don't pay more than $2 a gallon. so For a 55 gallon tank with everything I wouldn't pay more than $110 but take into consideration the condition of the tank. For bigger tanks be sure to check the center frame beam if it has it and make sure it's not broke.
Also if your looking on craigslist try these words in the search bar for some results that others wouldn't show up. Also check the pet section and general section but for keywords search for.
- Fish Tanks
These will usually touch on everything that you'll be looking for.