Pegase is a one-stop shop that seems to have a monopoly on most of the tourist business around the great dune. Of course you could probably arrange things on your own with any of the tour companies, your hotel or one of the freelance guides who will likely introduce themselves to you around town or near the great dune. Most people arrive as part of a large group and everything is prearranged.
Part of the fun of a camel trip into the dunes (if alone you should be able to negotiate a ride for around TD10 per hour) is being outfitted in a long Berber style tunic. A lap around a desert track on a go-cart is around TD10 but for a much more exciting ride in the dunes it's a whopping around TD60 for 30 minutes. To view the dunes from on high, though not too high, a 10 minute flight in an light plane is TD60.