I do the type of "sessions" you are talking about with friends and co-workers.
I take a very different approach to my sessions and fees. Namely, other than some reasonable token of appreciations, I refuse to accept money for my "services." If someone wants a large number of prints, I do have to charge but that has not happened so far.
The rational behind this is that, personally, I am still learning and this is my hobby. I do not want money involved in it. It takes the fun out of my hobby. Also, when money is involved, there are certain level of expectations. I simply do not want to deal with that.
This comes from someone who turned every one of his hobby into a paying job in the past.... (and lost the hobby)
Your "fee" sounds reasonable but I thought I'd throw in how I deal with my own situation.
By the way, my "clients" will not go to commercial photographers otherwise, so I am not taking a paying job away from any working photographers.