Some melt down over the awful resale values of desktops. Part of that is how much I can build for cheap.

Example? Read https://www.reddit.com/r/PCMasterRace/wiki/builds

If you want a price the two methods are:

1. See what it sells for on Ebay.
2. Take the price you bought it for and halve the price for each passing year.

So for a new 1000 buck PC, 500 in 2015.
250 in 2016.