Campaign world dependant.
In one game, my DM and I settled on (for NPC mercenaries specifically) (Level^2) * 3 sp/day, and that I was paying them a full month's worth at the outset (as well as trail rations for a month, plus a little for inconsequentials; as a mercenary band, they came with basic equipment). Mind you, the King's Eleite should definately be well paid - as was mentioned, they should be landed with a self-sufficient manor (suitable for taking care of their families, livestock, whatever as necessary, perhaps also generating a side income in a good year), equipped to the King's standards (masterwork armor/weapons), dressed in the King's Livery (call it a Coutier's/Noble's outfit - several, as you can't have them coming to work dirty - when traveling, have at least one or two dressed exactly like the King himself, to help foil assasins that aren't spotted), eating at the King's table (have to test for poison, if nothing else), and sleeping in the King's Castle (must have at least one, preffereably two or more, awake and watching the King at all times, and have even more within easy shouting distance). This should all be in addition to a wage of some kind, by whatever name.
And do note that most of the stuff the King gives out goes away if the bodyguards fail in their primary duty - bodyguards who lived and failed in their task become presumed traitors - which means, among other things, that lands are confiscated by the heir (bodyguards who perish defending their liege need not worry on that score, for the most part), any family is taken captive (potentially executed).
