The big site cost is paying for memory, because we currently load everything into memory. The SQL database doesn't directly fix that, but it makes it possible to load data as-needed. Once we actually do that, we'll be able to pay for hosting with less memory.
It's not quite as easy as it sounds. Changing a data field to load from SQL where it's needed is easy. The problem is, the code currently assumes data is cheap, and will do things like load that field for every post ever, just to get it for a single post. That has to be fixed. But we'll get there.