yes and no. the difference in cost between the weapon and the countermeasure matters, but it's not the whole story. the value of the thing being defended also matters. in the absence of a viable alternative, it's easily worth burning $1-2mm per interceptor to prevent a mission kill on a $2bn destroyer or a strike on a major population center.
not saying this is you personally, but people seem to view this kind of thing as being similar to RTS, where players start on even footing on a symmetrical map, and games are won or lost by stringing together resource-efficient "trades". real world conflicts aren't like that. western countries have relatively deep pockets and low tolerance for casualties (and collateral damage to an extent). as a result, they are willing to pay a premium for training, survivable equipment, and precision munitions. it's a different doctrine altogether.