Bodyguards are privately paid by their masters (your FM) except recruitable merc generals, and in EB timeframe, every noble men (your FM) often has several farms, industries, and lands at their exspense.....

But I said they are not most cost effective.... all you need to do to lost them is a simple cavalry charge on their left side (who often result in death of their masters), but that's true that they are "low-tech" elites.......