I use zetaprints.com. They charge you by the jobs that you actually receive. It is less than $2.00 per.
Fairly basic controls. Not great for really in depth design.
However, I use it a lot for Corporate style stuff. For example Real Estate offices where I am just changing name, number and website for an agent and the basic layout and logo is always the same.
The end user basically fills out a form and it spits the design out. You set all the parameters beforehand.
Not sure if this will be helpful or not, but worth taking a peek at.