I've done a 12 month plan, and this dictates my price for wedding and portrait sessions.
Firstly, I determined ALL my costs: insurances, business cards/flyers etc, advertising, website, professional membership fees, car running costs, telephone and electricity costs, presentation boxes, discs, prints, office equipment, gear upgrades and maintenance etc etc etc.
Then I estimated that 75% of my costs would be sitting with weddings. So I totalled up all my annual costs and split it 75 weddings, 25 portraits.
So let's say for example, my annual cost is $10,000. Therefore my annual cost for weddings is $7500 and for portraits is $2500. I then estimated that I would do 3 weddings this year and 10 portrait sessions.
So now I know that, each wedding I need to make $2,500, and $250 for each portrait session just to break even. Do I set these as base prices? For Weddings, pretty much, have 4 weddings locked in, so I'll make a profit this year. For portraits... no, because if I tell someone they need to pay 250 upfront before they receive anything, they'll tell me to jump! So I accept some risk and set a modest sitting fee and do my best to produce great images and sell some prints etc.
Right wrong or indifferent, this is my approach, and it's seen my costs covered and some profit to invest into some new gear, which as a part-timer with another full time income, I'm happy with at this stage