A secondary question is how much usable life would you expect out of a significant upgrade like in my example below?
I have a 5-year old Sony Vaio VGN FZ140E, which was a high-end laptop when I purchased it, not a mid-level or bargain. The specs are 200 GB hard drive, 2 GB memory, a bad battery, and Windows Vista, stable but corrupt enough not to update since February 2012. It works fine and currently I’m just using it to browse on the nightstand, mostly because of the dead battery.
I believe a nice upgrade would be a 250-ish GB SSD, at least 2 GB more memory, a battery, and Windows 7, all of which would amount to roughly $500-$650 depending on how you source parts. Such an upgrade would make it usable for mobile computing again at some level.
Can you reasonably put $500-$650 into a 5-year old laptop and what do you get? I know that resale value will not increase or increase by a tiny amount. Note these models were also known for getting hot, especially batteries (some spectacular failures).
I’ve also noted no one seems to discuss the upgradability of cell phones, and upgrading MacBook Airs seem to be a non-topic anymore.