Neither, really - a lot depends on the size of the pot or plant itself - if its a larger, more mature one, it'll be more expensive. It is often more expensive to buy a rose that isn't common or popular though.

The less expensive ones tend to be bare root, which has advantages and disadvantages. They have a slightly higher mortality, but they make better plants in the long run, since they can adapt more quickly to their site. The very cheapest of the bare roots, though, tend to be short on roots and/or canes, which adds another year to the time they fill their role in the landscape, at best. The most expensive ones tend to be either the newest varieties, or the old kinds that aren't produced in job lots anymore.