Ultimately in a free market society, the true answer to this questions is one we should ask ourselves.
Would you ever eat at mcdonalds if they were charging sit down restaurant prices for their food, because if you increase the pay the food prices will inevitably increase as well. Mcd's value is in its value, you not willing to drop $20 on a bigmac then you essentially agree that fastfood workers don't deserve a living wage.
European McDonalds have almost the same prices on the menu as the American ones, and they pay significantly higher over there.
No, a Big Mac or 20 piece McNuggets aint gonna cost you $20 in order for workers to make a livable wage where they can get shelter and food.