If you're looking for Disney souvineers, don't spend a lot of time shopping around for prices and variety of items. Other than "theme" items like Mickey in safari gear, you'll see the same things in every store at every park. I found the biggest variety at Mickey Gear at Epcot and a similar store at Downtown Disney.
What to buy: The quality of stuff is good and there are plenty of things to consider, from magnets to pens to coffee mugs to tennis shoes to hats to watches, and of course t-shirts and sweatshirts. Even things for very tiny babies.
What to pay: All in all I'd say the prices are fairly reasonable -- you ARE at Disney so expect to pay a little more for something Mouse related, but the stuff is fun and unique so splurge a little for that cute tank top.
We browsed some in the shop that was in the Cirque de Soleil building
What to buy: music and DVDs, one-of-a-kind artwork, apparel and toys.