'Sides, if mercs are actually more costly than levies or militias is anything but cut and dried. After all, you're just expending cold hard cash (and usually generous looting rights) to hire some foreigner to fight under your banners. That means you're not taking your own people out of productive labour, say farming, and as the French wryly observed in the 1500s, also means your potential enemies have one less man to hire against *you*...

Also, enrolling them as mercenaries was often the most effective way to keep diverse highland tribes and suchlike out of mischief (ie. raiding your stuffs for fun and profit) and make them useful...