In this day and age using your own domain adds no cost aside from the domain itself.

I suspect that using your own domain for large volumes of photos still adds significant indirect time and energy cost of fiddling around with separate front end and mass storage backend. A bunch of time spent tweaking to keep full images and smaller versions organized and quick while not paying a premium for storage is a cost, and those hours have value.

I didn't mean hosting them yourself (which there are plenty of packages for) I mean using your own domain via CNAME on a service like smugmug.

The price you sell something at should not be based on its cost to yourself, but rather the value you provide to your customers.