Potentially 100 million PSN accounts (PS3, PS4, PSvita combined) * 3 GB = 300 million GB. Or 300'000'000 GB. Or 300'000 TB. 300 PB. Yeah stuff the 1024 conversion. But I would think 300 PB to be quite substantial amount of space just for game-saves. Devided by 3 it's still substantial mind you, but they wouldn't need it all by day 1. Since PSN is evolving though and game-saves rather big, I do wonder how much space they really need to get by.
The big issue I see isn't the online space allocation per user; it's the game-save size itself. When I look at DriveClub and see a Ghost-save of 100MB, I get a WTF moment. First of all, this isn't something IMO that is a "crucial" save or something that belongs on a cloud. So if I was a hardware vendor and designed the services, I'd perhaps split up game-save in crucial and not-crucial - the former a save that is backed-up to the cloud, the latter isn't. Then as a vendor, I'd also put an upper limit to game-save sizes of a couple of MB. If you require more, you need a good reason for it. A ghost-file of 100MB just seems overkill. If you designed it with the goal of keeping the save to a minimum, you could probably reduce it to a sequence of controller-inputs (with a few synch-points along the way to keep everything in check). Of course, as a vendor like Sony, you can't tell your developers of other publishers on how to program their games. What you do is put up rules and limits to keep things in check. If developers knew there was a limit of say 20MB per game-save, they would probably invest more time in finding ways to keep their saves below that rather than just doing it the easy way and without worry.
From a consumer perspective, you don't want unnecessary uploads to cloud on a regular basis either. Just imagine everytime you play DriveClub and save a ghost (which it does automatically anyway) and at the end of it, you always have a few hundreths of MB upload to secure. Ridiculous.