Green’s Dictionary of Slang

fold n.1

[powdered narcotic drugs are sold in folds of paper]

(US drugs) a piece of paper folded to contain a measure of a given drug; usu. $25 per fold.

[US]Bentley & Corbett Prison Sl. 74: Fold A paper that is folded in such a way as to keep the drugs it contains from falling out [...] A standard price for a fold is $25.