Just a couple of thoughts:
It's probably obvious to you that the resources used for backup depend a lot on the specifics of the site you're running. If the site is primarily staticly published, you could imagine that no backups are necessary since your "backups" are in source control. The same would go for the case where your site is a server application (except for the data). Regarding data from a CMS or various and sundry items entered by or uploaded by users, by definition if you're running efficient incremental backups this must be less than the bandwidth used by your site itself otherwise how could the data have gotten in?
Another consideration here is discovering how bandwidth usage is implemented. For many web hosts, only http traffic is measured. I'd guess that this wouldn't be the case for a dedicated server because of the wide variety of network-listening applications that you could host on it.
Remember that the dedicated hosting is actually provided by Alchemy Communications. Dreamhost in this case is acting as a marketer for Alchemy and Alchemy is fulfilling the service. Here is a link to the Alchemy dedicated hosting FAQ:
Note that the FAQ does mention the availability of onsite backup starting at $30 a month for 30GB.
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