Awesome write up Phixer. A lot of really good points made, but you cant follow your pricing with larger tanks. There should be a formula in place for larger tanks. A kind of compounding formula for prices exponentially increasing in value. Something like Price=(gallons)x(1+ratio of increase for each gallon in decimal form/gallons)^gallons. Something close to that, but you get what I'm saying. You cant calculate a linear amount of increase of $ per gallons, there has to be the compounded amount. Then in the end, multiply a fraction of the total, based on what the condition of the tank is. Say the formula comes out to something like 435 for a 10/10 used 300 gallon aquarium, just simply multiply that by x/10 for the condition of your current tank.
In theory it seems pretty ideal to me. Just need someone with the brains that can think up the proper formula.