Too many variables for a “rule of thumb”. The world of digital components is still on the move, with new products and manufacturer upgrades of existing products coming out that make certain existing products less desirable and therefore less valuable.
Age, popularity, reputation of the manufacturer, market saturation, and more can all affect the going rate of used products. I always check HiFi Shark (you can also ck A’gon Bluebook), which shows the listing prices (not the actual sale prices) of the items that were sold, in order to put myself in the ballpark of how much I should consider spending. The market saturation is important to know whether the item is something that comes up for sale every week, or once a year. If a certain item does not get listed very often, you need to jump early and/or pay more or otherwise you might not have another chance for a while.
