Hobbyists I work with typically start negotiations at $1 a gallon for glass and $1.25-$1.50 a gallon for acrylic. If you have a stand, add $0.25, and if it's inclusive of filtration, heater, decorations, etc., add another $0.50. The lighting will add a little more, depending on whether you're running fresh or salt... Instead of selling outright, you might be able to find a trading partner who is looking to upsize, and your need to downsize might create a perfect trade with $$ opportunity...