Right, so the price of a used bat is purely based on how much it cost new minus how much it's been used?
No. The used price of a bat is more complex than that. It's what it cost new, less how much it's been used, multiplied by rarity factor (which can be a requirement) and a desire to own factor.
On the pure logic of the statement, a GN dynadrive in good used condition should go for £20 max, yet on ebay they go for £40-£70, which is ridiculous given age and likely use.
If it is a recent model, that's a relatively fair statement though.
As for custom made bats; if they are custom made for you, then second hand they will be worth less to someone else, in my view!