How much should i charge on a building site?

I am a recently self employed carpenter. Private work is pretty slow so i was thinking of going on a site. i would be going for the main contractor. The site agent told me that they have already employed timberframe specialists to do all the first fix, but the contract for the second fix is open to offers. how much to hang a door? how much to fix skirtings? with private work i would charge by the day, but a friend said that its different on site and you have to be more competetive? Im a bit bewildered really? any help would be much appreciated, thanks.