It really would depend on how much square footage it would be, plus, the material you want for your roofing. Sometimes roofers can give discounts for bulk when they do a lot of work, so the square footage might be different for that specific job. It sounds like you have a smaller area of roofing to get done for the deck, so there's another possibility that a roofer would give you a discount for a smaller area too because it's a quick and easy job for them. Compare at least two roofing companies as well. Hope this helps, Armandochave!