I fully agree with what Jamie posted.
I need to point out that one shouldn't use the multiple-sites feature to host websites for multiple clients. The reason is that a client may decide they want to move to different hosting or their site grows to require dedicated hosting. There is not an easy way to extract one site from all of the others which means if the client is given the database containing their site, they will also have the data for all of the other clients using that database. This is not a good thing at all.
The Multiple Sites feature was developed to make it easier for a site owner to host multiple sites with one code-base but not to necessarily make it cheaper for developers to host multiple client sites.