Expect to pay from £250 for a quality, portable pizza oven right up to £2,300 and even more for a permanent pizzeria-style brick and clay permanent wood-fired oven. There are cheaper models available but these can sometimes be a false economy as they may not be built to last.
Look out for what is included with the pizza oven. All will include the stone and oven but you will always have to buy fuel separately, so factor this in. Often multi-fuel ovens require you to buy the gas attachment on top of the cost of the oven, too.
Some pizza ovens come as a kit including a peel and thermometer. These aren't essential but it is much harder to get the right temperature without a thermometer and very, very hard to put a pizza in an oven without a peel, so see where they are included. Finally, a cover is a nice extra. It will mean you can protect your oven outdoors over summer.