If you are doing this only as a hobby and you plan on keeping your day job you need to still be charging what is consistant in your area. I had the same trouble a few years back and soon realized that I was way cheaper than "real" photographers out there. But you will say "I don't have a brick and mortar and no insurance and my tax impact is minimum so why do I need to charge more?
First what is your time worth? Did you buy Photoshop? Did you take classes to learn it? or are you spending hours upon hours to learn it and make it sit up and bark so people think you know what you're doing... What is your time worth? My time is very valuable to me as it is to most of us. I'm a NAPP member and that's 100$ per year plus my internet access (49$/month). If you think about it you have many costs that need to be covered by your "activity". So look at what your costs are and charge accordingly to cover all those costs..