This is a tricky one.
I also travel and I use travelling costs to cover these outgoings.
I have been a trained teacher for over 12 years now and have taught peripatetic lessons on and off for around the same amount of time.
I am thinking seriously of increasing but am fearful of losing students so I think what I may do is wait one more year and point out that I will be increasing. Any potential new students I have in the coming year I may consider bringing up the cost. It will probably be about £2.00 increase.