A small shop is available on the boat. Here it is possible to buy souvenirs, national Egyptian clothes, and jewelry. I have bought for a night-party a Nubian suit consisted of three subjects: a long shirt, a jacket and a turban. It costed only 60LE (12 dollars). You may see me wearing this suit in my travelogue "Night-party".
However I would not advise this shop for more serious purchases. For this purpose it is better to take advantage of shops on coast.
The stalls outside Kom Ombo are good places to pick up something for that fancy-dress party!
What to pay: More haggling!