Back in 2010 a HiSeq 2000 cost $690,000 - you'll be lucky to get anything today. A V4 2500 might get you $100+k, but your V4 capable V3 just $20k. Depreciation sucks!
In accountancy, depreciation refers to the decrease in value of an asset over time. Most of us come across this when buying a car - for me that's usually on the nice side where I'm wondering why someone is selling a car they bought five years ago for so little!
But no asset lasts forever, and certainly not in the world of NGS. That "faster than Moore's law" graph you see at almost every NGS conference has a dark-side, and instrument depreciation is part of that. If this depreciation were accounted for properly thent sequencing costs in most labs would jump due to the relatively low number of runs performed.
Incidentally if you have a MiSeq sitting idle then these are pretty hot right now - I can probably put you in touch with someone who wants to buy one if you are interested!