c. 1200, "price, value," from Old French cost "cost, outlay, expenditure; hardship, trouble" (12c., Modern French coût), from Vulgar Latin *costare, from Latin constare, literally "to stand at" (or with), with a wide range of figurative senses including "to cost," from an assimilated form of com "with, together" (see co-) + stare "to stand," from PIE root *sta- "to stand, make or be firm."
The idiom is the same one used in Modern English when someone says something stands at X dollars to mean it "sells for X dollars." The meaning "equivalent price given for a thing or service rendered, outlay of money" is from c. 1300. Cost of living is from 1889. To count the cost "consider beforehand the probable consequences" is attested by 1800.
In phrases such as at all costs there may be an influence or echo of obsolete cost (n.) "manner, way, course of action," from Old English cyst "choice, election, thing chosen." Compare late Old English alre coste "in any way, at all."
"be the price of," also, in a general way, "require expenditure of a specified time or labor, or at the expense of (pain, loss, etc.)," late 14c., from Old French coster (Modern French coûter) "to cost," from cost (see cost (n.)). Related: Costing.
late 14c., "serving to effect the intended purpose," from Old French effectif, from Latin effectivus "productive, effective," from effect-, stem of efficere "work out, accomplish" (see effect (n.)). Of military forces, "fit for action or duty," from 1680s.
"tried, tested; experienced, expert; reliable, effective, trustworthy," late 14c., past-participle adjective from approve (v.).