Commodity hardware has gotten very cheap. Hence it often makes more economic sense to spread the load in the cloud over several cheap, commodity servers, rather than one large expensive server.
Microsoft's Azure data pricing makes this very clear. One Gigabyte of SQL Azure costs about $10 per month. Azure table storage costs $0.15 per GB per month.
The data transfer costs are the same for both. With Azure table storage you pay $0.01 for each 10,000 storage transactions.
To break even with the SQL Azure price you can get about 9,850,000 storage transactions per month. That is a lot of bandwidth!
Another way to look at the cost is to suppose you need only 2,600,000 storage transactions a month (1 a second assuming an equal time distribution over the day). That would cost you only $2.60. That means you could store almost 50 GB worth of data. To store 50 GB worth of data in SQL Azure would cost about $500 / month.
If you don't need the relational model, it is a lot cheaper to use table or blob storage.