In the late 90's we'd often get customers wanting to trade in their old bike for the latest & greatest machine. At the time it was mainly MTB's as this was a boom period. As a trade in, that the store would have to sell, we came up with a format that allowed shop manager or Saturday boy a rough calculation to work on, even if you hadn't seen the bike.Technology was the driver for this format & hence the calculation was brutal. For those not riding at the time it was like a MTB arms race, each year the manufacturer bringing out better, lighter, more this & less that. I think the MTB in that period had an advancement injection not seen before & I'd doubt if we'll see again. So to the formula.
Normal retail price / 1/2
Then for each year old it was 10% would be taken off that price.
Of course there was exceptions as condition of the bike influenced whether it would decrease rapidly or if it would approch the 1/2 New Price.
So is there a solid formula for working it out then? My answer is no. This often causes a raised eyebrow when scouring the pages of the Comic because of this. It's probably the reason why I've often undersold items, & why eBay is popular with cyclists as the end price ends up being a true (or truer) price.