How much is a normal amount of silver for a lvl 30 merc?
Normally zero. Most people who hire ignore this unless a note to be paid is in your application. It's not a very productive way to make money, frankly, even if you do get paid 20-50 silver. Going back and sending silver out is usually more of a hassle than it's worth.
If you want to be a "merc" in Strat, building up an army with soldiers and gear (earned by trading) and then offering THAT is really the way to go about it. Not fighting in battles and getting paid.
Or, you can merc for cRPG gold. But that's not especially common either.