There are a lot of shops of all kind on top of Mount Titlis : I saw a shop for fine watches and a photoshop, where you can dress in traditional costumes and have your photo taken, there are seminar-rooms as well as various bars, so you will not get bored in case of bad weather on top of Mount Titlis.
What to buy: They really have a great choice of Swatch- and other Swiss watches
What to pay: Sorry, I have no idea
Just off the main square, the COOP grocery store can provide you with just about anything you need for a picnic. It's not too bad a view as you walk to it, either.