Howard - you are right to be careful of house PA unless you know it. Also it helps to have an additional source of income.
How do you calculate what to charge? I tend to charge a basic amount - less for just me and a box, more for when its the full band, then add a bit more on if its over 20 miles away. However, its all a bit ad hoc. I always write down how much I've quoted on the booking form immediately.
If you stay local, and are doing an open fund raising charity, then you can e mail those people you know who like barn dancing. This means you get more people to the events - a bonus for the charity.
FloraG