My journey to work is 5 miles, a colleague lives 5 mins walk away from me and I don't drive. I walk to her house and she gives me a lift there and back most days when we are both in. (On average 8 journeys per week).
It works well, we're both happy and it's good discipline for both of us to make sure we leave on time! She says she is very happy with what I give her towards her petrol and car costs.
So I am just checking/benchmarking what you think I should reasonably be paying for this wonderful chauffeured taxi service.