There is a really good book on film budgeting called "Low Budget Hollywood," sorry I don't have anymore details than that, as I gave my last copy away to another film maker as a present.
A budget is really just a slightly more complicated shopping list, you write down everything that you need to make the film, actors, tape stock, costume, lights, locations, catering, insurance, props, editing, soundtrack, contracts(legal) etc and then you find out how much each of them will cost.
If your actors are working for free, then you put a zero next to actors on your list. However, free might mean you have to feed them and provide transport, so you have to put an estimated figure against those costs.
When you've done all your research, you add up the figure and then add another 15% on top. That's your contingency budget, the budget you go to when your free lighting kit breaks down and you have to hire one in.
The trick with budgets is to make sure that you get every, single, potential cost nailed down and that means having more than one person do the script breakdown with you, to make sure that you haven't overlooked anything.
For me, on a lo/no budget film, the key expenditure is always catering, petrol and insurance, pretty much everything else you can hussle for free.
A well fed crew will be more committed to the project and won't bail half way through the shoot. Having proper insurance means never having to hand your house over to the court.