It means that while they get cheaper over time, they take longer to loose enough value to be cheap.
And then their value follow a bell curve up, when they getting sparse and therefore become collectible.
Heck, there a still PPC macs sold for up to several 100$.
Intel Mac minis reach 500$ for a corei7 from 2012. And as we are no longer in the 'testing period' of MorphOS we cannot seriously recommend a CoreDuo machine, can we?