In my case, I'm paid an hourly rate. Depending on the distance, hours, I'm compensated for these as well. The studios I shoot for have a lot of work, but their packages are low so if I went with a percentage, I'd make even less.
You'll just have to talk to them to find out what exactly they are willing to work with, what their packages are at and go from there.
I am "shooting" to go out on my own next year though. More freedom & creativity and not as burdened with certain "must-have-shots" or shoot-list
