Well, a data center using lots of drives can probably use more of the space on them. It wouldn't be unusual for an individual person only to fill up 1/10 of his hard drive. It would be less common for a data center to have 10x overcapacity.
If they wanted to be really clever, they could also analyze the data when it's uploaded and encode it in a less redundant way... for instance, it's quite likely that large numbers of people will have very similar mp3 files... if you could figure out that 99% of a certain file is exactly identical to 99% of another file, why store that data twice?
But, that gets very complicated... you're just shifting the hard drive costs into developer and cpu costs... so it may not be economical.
Clouds must markup. They can't charge the same prices or they are merely covering hardware expenses, which doesn't cover personnel expenses, let alone profit.
Fortunately you can get some premium value out of that markup that is way more expensive to set up yourself. (In particular multiple backup sites.) But the cheapest storage solution absent all other concerns will always be simple local storage.