You will not starve without PSs. You may, however, never see a waitress or avoid having to stand in line for your food. Its often possible to walk into some restaurants, even during busy times, but if you want to be assured of sitting down - particularly at a specific place, book a PS.
Less expensive - we actually found every place we ate to come within $10 for four adults and two children - except Narcoosee's was more expensive. That was Biergarten, Teppenyaki, Le Cellier, Boma, Marrakesh. The trick to less expensive is above - book restaurants where you can order off the menu and choose less expensive options - the top and bottom of the menu is within a few bucks at all the "moderate" restaurants. Split meals (or at least courses, its unlikely everyone needs their own dessert). Don't drink alcohol, and if you can, stick to tap water. Sit down at lunch, the menu is usually cheaper, and have your counter service meal for dinner (those prices don't change).