Yes, there are cheap options for wood-burning ovens now. If you’d rather get more mileage out of that large grill you have in the backyard, though, then pass on the oven and pick up one of these Baker Stone oven boxes instead.

A pseudo-oven, it sits on top of large grills and uses heat from it to cook food items with the same efficiency as a wood-burning oven. It works fast, too, harnessing the heat from the grill to the high temperatures required to cook an entire pizza in as little as two minutes.
The Baker Stone has a housing cut in enameled steel, with a stone baking chamber serving as the cooking surface. A unique airflow system circulates the heat efficiently inside, ensuring every part of the interior is subjected to the same high temperatures for fast and even cooking. Do note, like conventional ovens, this will require some pre-heating time, so you’ll want to set it up and turn on the grill for at least 30 minutes before you start cooking pizzas.
Dimensions are 22.8 x 16.3 x 6.2 inches, so you’ll need to use it strictly with large grills (three burner gas grills or bigger). Aside from cranking out pizzas, you can also use it to roast meats, fish, and vegetables, as well as bake bread, pastries, and cookies.

Source: coolthings.com