It all depends on what "usual Disney merch" means to you, and what you're shopping for off-property.
If you want a souvenir T-shirt and some flip-flops, $100 would be adequate. Keep in mind that Disney stuff is EXPENSIVE, though -- the last time I was in a Disney gift shop I wanted to buy a pen for my kids' teachers, and I couldn't find one for less than $16!!! (You might want to check the Disney Store website before you go -- you can buy park merchandise on the site with coupon codes and free shipping, often for less than you'd spend in the park.) If you're buying Pandora jewelry and Dooney & Bourke bags throughout your trip, you could spend thousands.
My family falls into the "low-priced souvenir" category. We pick up a single T-shirt or hat for each person in my family of four - so $100-$120 is all we'll spend in a week's vacation at WDW (other than food).
If you want to calculate your expected gratuities, run your restaurants through distripplanner.com, click on the arrow to expand the dining plan calculations, and see what it's giving you for the tip amount (calculated at 18%, I believe). Or just check menus for the places you're going to visit. For my family of four, a typical Disney restaurant tab averages about $125 -- or $25/day in gratuities.
Other places you might find yourself spending cash at Disney: cabs (if you plan to take any), and baggage handlers ($1-$2/bag for your DME driver, Bell Services, or anybody else who handles your bags).
