Prefab sunrooms are often, but not always, cheaper than custom-built sunrooms. There are many different add-ons you can specify for a prefab kit, and a top-of-the-line sunroom kit with all possible options can approach the cost of a room addition. What tends to give the impression of prefab units being cheaper is that they are sold as materials only. When a prefab company quotes you a price, they are quoting the price of the sunroom kit only.