A good place to start is to know your day rate and thus hourly rate. Look in archives of the biz and marketing forum of the COW for an exhaustive set of how-to’s and discussions on how to set a proper rate.
Next is to manage and understand expectations, so you can estimate with some confidence what the number of hours will be. What are you wanting to offer, and what does the customer want? Some wedding clients want wall-to-wall documentation of every facet, plus separate edited highlight reels. Others want something much more scaled-down. Do they want a montage or something more real-life? How many camera angles? Any special effects? Coverage at the reception? The options are near-infinite. What are you prepared to deliver? And what are the “deliverables”? That is, is this final output a DVD, a Blu Ray, a digital file, a YouTube or Vimeo upload, what? If there’s physical media being given to clients, how many?
time x rate + markup = cost.
Typically, one should get a third of the estimated cost up-front as a deposit, to cover your costs. A third on the day of the event, and a third at delivery.
What you cannot afford to do is to set a package price with no margins for the extra time needed in post to deal with the inevitable things that can come up, like fixing audio problems, or cutting a complex montage or two that needs extensive color correction work. Set too low a package price, and you often wind up losing money on the deal vs. doing something else like flipping burgers. Worse, perhaps, is “low-balling” the price or your rate, which puts you on the record as accepting too little money for your work, and this is HARD to break out of once people know how cheap you did it for one guy.
So don’t ask what to charge, so much as calculate the figure at which all this effort and time and lost opportunity to do other things is worth it to YOU to do the project. If you can’t make a profit on it after expenses, it’s not good business to take on the deal.