As far as I know honey is the one and only eternally lasting source of energy.
Looking over some prices over here in Romania, an NRG-5 MRE pack for one day (2,300 kcal) is worth about 38-45 RON (so let's say about $10) while from only one 11 RON ($2.70) can of food you can get 1,000 kcal already, which means you can get 4,000 kcal for only 44 RON ($10.80).
But again, the big negative side is storage space and probably expiration date as well, unless you freeze that stuff. MREs are, I imagine, waay smaller than a few kgs of canned food. The point I was trying to make is, one can basically get canned stuff for almost half the price of MREs while also hitting the necessary daily calorie count if budget really is an issue.