We always try to pay our mercs, depending on who the person attacking/defending is, it varies some. Generally if you are a merc you are prioritized lower than people who sign up for free (usually friends of the faction or in other friendly factions). However, we usually need mercs, and pay according to performance or any pre-set agreements. If you ask a reasonable amount and do "ok" in the battle (somewhere in the middle) you usually will get what you ask for. If you do better, you will be rewarded with more money.
If I'm the leader of the battle, I will generally pay all people who sign up that are not in "friendly/allied" factions, regardless of merc status.