By Riley Gutiérrez McDermid The ‘pizza principle’ had held steady since 1980, but no longer. Every New Yorker knows one immutable thing about that magnificent city. While everything else may seem expensive, luxurious or out of reach when you’re barely getting by, you can always count on getting a slice of cheese pizza for basically nothing to keep you from starving during your Big Apple salad days. Well, not anymore. A new report has found that inflation has jacked up the prices of fuel, heating, ingredients and labor by so much that a plain slice of pizza now costs more than an average subway…