The formula for depreciation would need to consider the service life and material condition of the tank. Best to emulate and incorporate pre-existing principles such as used in Kelly Blue book for autos. I really dont see anything wrong with your formula. For acrylic I believe 30% to be fair, however each tanks service life can be altered depending on factors such as how many moves it has endured (increasing propensity for leaks thru weakened bonds) or exposure to UV light (increasing tendency for 2 part polymerizable cements to yellow). Some may chirp about 30% their logic may be equally founded.
I look at factors such as the age of the tank and method of construction. Solvent cement or 2 part cement, how many moves and of course thickness of material. Is crazing evident?
FOR NEW TANKS
The reason I say most tanks are over priced is because after subtracting the cost of materials, the cost of labor is realized. And it's out of this world... totally ridiculous. This is where the scam is, labor cost. At this point cost of labor needs to be compared to industry flat rate. The cost of labor then needs to be factored into the resale equation. The process of building an acrylic tank is not that difficult and in no way justifies the price charged. As in economics though, if people stop paying it the prices will drop.
These costs are passed along for used tanks. Because the original buyer got ripped off they tend to do the same when re-selling the tank.