How much should I ask for a 55 gallon aquarium?

Yes, if you had to buy all that stuff new, you would pay $350-$800 for it, depending on quality. However, there is simply no great market for used equipment, and I don't think he is going to get any takers at that price. Even on Craigs list you can find listings where that stuff is simply given away. Look at the equipment and if you like it offer him $100 and I think you bought yourself a fish tank. I gave my 100 gallon away to a neighbor after I got tired of the hobby.