From my experience, the locums charge to the group is WAY high, and I don't get the angle. For what they charge, there HAS to be a better way.
If you are looking for extra cash, as EM boarded, shoot for the moon. Give them a number that is out of this world. If they take it, then you know you screwed up ('cause you could have gotten more).
The locums takes a percentage off every hour. I gots Weatherby burnin' up my phone lines (which is kinda OK, because I don't have a land line anymore).
Remember, if it is desperate enough to need locums, very often, they're (the locums company) willing to drop in (and the ED take) some IM or FM burnout, who seriously suxx (I came in full time and replaced a locums guy that had been there almost 2 years, and it's like I am manna from heaven - I tell them I am a B+ doc, and they tell me they've never had it so good - I make decisions, and don't - literally - hide when there is a code, or pt that needs to be tubed).
And you tell the locums people to give you travel, car, housing, and food, and, if you can provide any of those on your own, then you tell them to put that into your hourly rate. Remember, they need you more than you need them. That puts you on the plus side.
Now, if you know all of this already, good on ya. As to the rate, call the director (if they'll tell you the facility, or area), and just ask.