Well a small convection toaster oven is pretty cheap and will allow you to make about 4 at a time. A mug press (like a geo knight DK3) will cost much more but can produce a finished mug about every 4 mins. Great for small orders.
We have used both, keep in mind an oven you will need mug wraps for each mug you place in the oven. You do not need wraps for a mug press.
They also make ovens specifically for mugs, but the cost is pretty high if your not doing a lot of mugs in a production environment.