'Cost' comes from Latin 'constare' (to stand firm at a price) — kin to 'constant.'
The amount that has to be paid or spent to buy or obtain something; an expenditure of money, time, or effort.
From Old French 'cost' (cost, expense, price), the noun derived from 'coster' (to cost, to be priced at), from Latin 'cōnstāre' (to stand together, to stand firm, to be settled, to cost), a compound of 'con-' (together, with, emphasizing completeness) + 'stāre' (to stand), from PIE *steh₂- (to stand, to be firm). The same root *steh₂- produced 'stable,' 'state,' 'static,' 'statue,' 'constitution,' 'substance,' 'establish,' 'instant' (that which stands upon the moment), and Greek 'histanai' (to cause to stand, whence 'system' and 'apostle'). The semantic development of 'cōnstāre' ran: 'to stand together' → 'to be agreed
'Cost' and 'constant' are siblings — both from Latin 'cōnstāre' (to stand together). What is constant 'stands firm' without changing; what something costs is the price at which it 'stands firm.' And 'constable' is from Latin 'comes stabuli' (count of the stable), built on the same 'stand' root — the officer