There are many factors that go into the pricing of used tools. However, the original purchase price is not, and never should be one of them.
Is your used tool the same as what is currently available new, or has the new model been improved in some way? Maybe your old model is actually better made than the current model?
Did your used tool come with a warranty? If there is still time left on it, is it transferable? Is there any real value in that warranty in the first place?
How used is your older tool? Is it older but yet unused, lightly used, still usable or about all used up?
The price you originally paid for the tool is irrelevant. Prices change over time due to inflation, and you need to compare what you have to offer with today's prices. Yet, if you just recently purchased a new tool and it is still unused, you cannot expect anyone to pay you what you paid -- you'll need to offer some incentive for a buyer to purchase from you. There simply is no formula that you can apply to determine a price. You have to evaluate the present value of the tool you have to offer.