Have you ever charged anyone for photography before?
If it's for charity/fundraising it's usually 50% off the regular rate. Just running a business costs a lot. Insurance, equipment costs, license, etc. so I'd have to charge a lot more than someone who's just getting paid to do it as a hobby. I'd probably give your employer a break and give them a discount. I do that too with my current 9-5. I've been hired by the company to do a couple of our corporate events this year, so I gave them a small discount. Free drinks, free food, budget for assistants and whatever I need to make my life easier. Can't complain. Oh ya...photography is a part time job for me as well, so full time peeps charge a little bit more than I do.
If you want more info, PM me.