I actually found a 1999 iMac set out for a special rubbish pick up day. The owner of the house was there so I chatted a bit, asked her if she minded if I took it.

It had last been booted on it, complete with working hard drive an; MacOS X 10.3. So $1.299 -> $0.

That's just typical for computer hardware though. Depreciation is 3-5 years normally, after which it's considered worthless.