well, I would say if the rules are basically the same as Mordheim, then you should calculate rating the same as you would in Mordheim (5xmembers, + total xp, + 20 for "big guys", + hired swords).
However, Mordheim's rating system is imperfect, since it doesn't consider equipment in the equation. So you'd probably want some way of figuring that out. However, you can't really just add equipment cost, because costs often aren't balanced (a guy with heavy armor, for instance, adds 50 to your rating! That's almost 4 zombies). So that's a bit of a pickle.
I'd say your options are to either go through and fix equiment costs, or find a more representative way of including them into warband rating. There have been a couple of discussions on exactly that in this very forum, so if you take a peek down the page you should find it.
Unfortunately, I don't have any firsthand experience with the TC 14 Pit Fight rules, so I don't know if there's any correct way to do rating.