A good starting place
Break it down by hours first. 8 hour job? 4 day job? Figure how much you wanna charge per hour/per day. Let’s start at $15 an hr. and say it is a wedding you are shooting, just you, and this is your first gig ever. The bride and groom to be want a two day shoot. The day before you grab exterior/ interior shots, people coming in and set up. Then the day of you want to capture people coming in, sitting down, mingling, the walk down the aisle, the kiss, the reception/ceremony, cutting the cake, people interviews/talking heads, etc.
Let’s say you have one camera. First day may take 3-4 hours, just getting b-roll. But the main day takes a full 8-12 hours. You have to get in on the action, but stay out of the way, transfer footage, make sure you have enough battery power all the time, and have decent sound quality for talking head interviews with guests.
Then you have to edit the footage. That could take anywhere from 3 hours to a whole month. But let’s say it takes 16 hours (2 eight hour days).
You could keep track of your hours worked and bill after the fact. Say you work 16 hours total =$240. This is your minimum. No equipment costs, transportation costs, labor, etc. etc. whatever. You could add on those fees after or even before. Let’s say you have a flat fee rate of $150. This takes care of everything else like utilizing equipment, ensuring you have extra memory cards/drives, batteries, etc.
Now you just bumped up your price to $390. You could put the editing fees in with your flat rate fee amount or even charge that additionally. If so now you have $630 you can tell the client.
It’s really up to you to see how much they are thinking as well. Since this is (I assume) your first gig, Just ask them how much they thought it would cost. If they say $200 and you were expecting that $240 or that $390 or even the $630, then you should figure out if you can do it or not for which price.
It’s all up to you and how much you think your time is worth verses how much quality you can actually produce. I suggest taking that low offer for first timers. You gain experience and see how the gig runs so that next time it goes even better, plus you will mess up somewhere, everyone does, it just takes practice and knowledge to work around the mistakes.