Hello Roro
OED says about the etymology of "cost" as follows.
a. OF. coster, couster (mod. coûter) = Pr. and Sp. costar, Pg. custar, It. costare:—L. constôre to stand together, stand firm, abide, be settled or fixed, stand at a price, cost, f. con- together + stôre to stand.
The construction...