I would have a call out fee that's based on how far I have to drive, and then I charge €35 per hour programming.
Do you allow your clients to access the design or just provide the compiled app?
Never give them access to the designer. Only ever give them the compiled application that does exactly what they asked for. I don't even tell my clients what language I use.
All they need to know is that I can quick develop exactly what they want with no fuss. If your pushed to tell them what language your using, I just tell them I have a developers licence for ADB, which is a cross between JAVA and VB.
Which is technically true since Brilliant Database is driven by ADB and VB.
You will find that the less they know, the more they will like you because you simply develop what they need without any fuss. You will also find that overtime you make more money off future updates and add-ons they need more so than what you would have originally been paid for writing the program.