C-4 stands for, Composition 4. it was the Fourth in a series of similar explosive mixtures adapted from the British standard plastic explosive used in WWII. Comprising of 88.3% RDX, 11.1% oily plasticiser and 0.6% Lecithin. the plasticiser contains mononitrotoluene, liquid dinitrotoluene, TNT,nitrocellulose and dimethylformamide.
C-4 has undergone several changes at various times to overcome brittleness at extreme low temperatures but it is basically the same mix in varying proportions down to 80% RDX + Plasticiser.
It is not "Cheap" in any sense of the word.
Source: Department of the Army and Air Force Technical Manual 9-1910 Order 11A-1-34