Ok here is some guidelines i always use so i get lots of prisoners and lots of gold :3
Pick a faction you want to be a merc of or plan to be with for quite some time, follow their armys around and join the fight that are pretty obvious wins, or help them out.
take the lords prisoners and all that comes with it, and reqruit all the prisoners they had on their end so that you can get some high lvl troops.
once the gold start clinking in on a daily basis because of how many lord ransoms you get "Counts,Barons,jarls" sometimes gives 20k gold for ransom, woosh :3
but be careful about who you try to ransom and capture, always keep an eye out who owns what city so you can build later without having to spend 100 days trying to get them not mad at you.
and once you got all the enterprises going, just sit there with your mercanary for a while and keep doing this, until you have a few 100k gold, "i did this until i had 1 million denars, and it was well worth it once i started my own kingdom"
So that's usually how i get gold in this game at prisoner department ;P