I just installed Google Drive, and I'm thankful that I had previously purchased storage for Picasaweb, even though I hadn't really needed it. Just look at the difference between my old storage price plan and the "upgrade" plans in the picture below. Previously $5 would buy a whole year's worth of 20 GB storage. Whereas now, $2.50 pays for 25 GB a month. Now I wish that I'd purchased 100 GB at the old price. According to Google's information on storage plans, the old annual plan remains in force forever, unless I let my credit card information expire or in any other way invalidate or change the current plan. So I had better beware.
The ups and downs of storage plan prices are interesting at Google. Prior to my 20 GB for $5 plan, I was paying $20 for 20 GB. Then Google reduced the price to a quarter of the original cost, meaning that for a while, before adjusting the Gigabytes, I had 100 GB for my $20. Now, 100 GB costs $60 per year, or three times what it cost just a week ago.
It's not quite as simple as I made it appear above. There have been other adjustments to Google storage capacities, and what counts towards storage (for example photos under a certain size are now free - but they have been so for the last several months).
Information on the storage plans :