With a 20MB block size limit you'll need a full TB per year if the network ever runs at full capacity. With a 1MB limit we're talking a more manageable 51GB/yr.
A 2TB HDD is selling at around US$70 currently. That's $0.096/day, or $2.92/month. And don't forget the cost will go nowhere but down.
Don't forget also that it will takes us a lot of years before each block is 20MB big.
I'd probably say 5+ years but that's just my guess. Memory and bandwidth will be much cheaper then than it is now.
Also it seems that nobody notices the pruning feature that we're going to have soon.