This is an awful guide to use, I guess, but here's where retail prices are at the moment on newegg:
Cheapest 40GB: 42.99
Cheapest 60GB: 46.99
Cheapest 80GB: 56.99
They don't stock 20GB ones any more, presumably because they're just not cheaper than the 40GB ones. 40GB ones may well start disappearing in the medium term looking at how 60GB prices are creeping down. I remember looking at these late last year, and the cheapest 60GB and 80GB drives were about $15 more expensive in each case. Looking at this it doesn't seem impossible for even 80GB drives to bottom out by the end of the year.
It's something I never thought about before, but it is amazing how we're literally using HDD capacities to define SKUs - the 20GB PS3, the 120GB 360 etc. In PCs, HDD capacity is the last thing you use to define its value..it's taken for granted that they get bigger and cheaper quite quickly. That characteristic, coupled with the now established norm of labelling SKUs according to that, could do some interesting things. It's quite crazy to think that Sony could effectively price drop or even increase the value of their system without it making any difference to their bottom line. Ditto with MS, you would think they could add more capacity to every premium in the morning without it changing much at all. I never ever ever thought about this before today, but it's quite a powerful mechanic at play here when it comes to pricing strategy.