Well, Apollonia, if you are going to house parties, of course I can see that it is a trip that you will have to make. I think what I would do is take whatever is the national average for a gallon of gas, and figure out 'on average' how many miles your car gets out of that gallon of gas. then from that number, just round it up to the nearest dollar for each mile, and calculate it that way. does this make sense to you?
if this is way too much trouble, then just figure something that sounds fair to you--such as an extra dollar or two for each mile. make sure you get good directions though, because it wouldn't be fair to charge them for extra miles if you get lost along the way.
however, years ago, my sister had a tarot reading party at her house, and the reader was coming from about 100miles away. this was in 2002, and though I cannot recall what the price of gas was back then, I believe she charged us a flat fee, because not only was she driving 100 miles, but it took her a good 2hrs. each way! We also fed her of course, and she got 2 or 3 breaks in between readings--and then my mother and sister packed up alot of the left over food in plastic containers to send home with her, (usual for mom), so she made out pretty well!