When you eat less, why does it cost you more?

a + a = 2a
a + a - a = a

But a > 2a.

Is it just me or does everyone who decide to cut down on their food intake find their grocery bills raking up a larger amount than usual?

Why does a salad cost more than a burger?
Why does organic food, which technically should be cheaper because it does not have the additional expenses of pesticides, be costlier? (Actually, I know the answer for this one. The whole taking time, fewer produce, better quality, blablablabla)
So here is my dietary dilemma:

It's too expensive to not eat. Buying vegetables, chopping them, packing them are too much trouble. Salads way too costly. Healthy items such as walnuts and almonds - haha - they cost atleast five days' lunch at McDonald's.

Am I going to find a way out of this?