Your belief has no basis on fact. Food prices are incredibly elastic, because there are readily available alternatives in terms of both venues and other food products. Inelastic products are things that people have no choice but to use (i.e. things they're addicted to or things that must be consumed and can't easily be replaced). This is especially true for fast food businesses that compete on price.

Simply put, a 20% hike in prices would lead to a greater than a 20% decrease in demand. You're thus expecting McDonald's to lose perhaps 5-10% in revenue, while increasing costs by 15-20% (labor costs are about 35% of an average fast food store's budget).