I know a few people who do this on the side and it really depends if you are shooting film or digital. Many have a flat rate per roll plus prints and others charge an hourly rate or a hybrid of the two.
If you are doing digital shots I would recommend an hourly rate and then cost plus 3 - 5% for the prints while you are starting out. Keep in mind normal portrait packages at a photo studio range from $1700 to $3000. I paid $2000 for mine but the same package today is $2500 and includes nothing additional.
To start out and get some business you may want to charge $100 - $150 per hour plus prints and guarantee you will take at least 200 - 300 shots of the wedding and reception.
I hope this helps, but alot of it depends upon the area in which you live and how much business you would like to profit.