I'm in a somewhat different situation, but my thought process as a buyer might interest you.
I was given a 2006 iMac a year ago. The hard drive was dead so I replaced it with a 1TB drive and have been using it as the computer that sits in the spare room. While we don't use it much, I've found more utility to it than I initially expected. Unfortunately, it won't run Yosemite and therefore the latest Safari, which means that Google Mail doesn't work half the time. To me, once OS support is gone a computer needs replacing, so I'm in the market.
My criteria:
I've got two iPads, two iPhones, two laptops and two desktops to try and keep current. It's come down to $/yr in my book. If an iMac has an 8 year lifespan, a 2008 iMac isn't worth much to me as it's probably going to be unsupported in a year or two. I looked at a 2011 that was for sale and here's how I see it. I can get a brand new one for $1,000 with an educational discount. At end of life, you can still get something for an iMac. Let's call it $200. At straight line depreciation, the 2011 is worth:
$200 + ($1000 - 200) / 8 * 4 = $600
That's straight line, meaning $100/yr depreciation. Some people are asking that, but then why wouldn't I buy a new one. Why would I want to lose $400 owning a 4 to 8 year old computer? Obviously, to me the depreciation should be faster earlier on, so I skew that number as I see fit, and that's what I'm seeing for prices as they range between $400 and $600.
As for your machine,
$200 + ($2000 - 200) /8 * 1= $1,775
Then adjust for the fact that you can get a refurb and faster depreciation, etc.
As an interesting side note, I'm not sure my pricing model reflects what goes on entirely in real world pricing, which is subject to market demand. People do seem to be asking more for machines than this would suggest and I will buy new if I can't get something in line with my formula. I can't afford to spend $500 on a machine that may only last two or three years.
