This is a can of worms on this one as I would have thought your minimum charge would depend on what you need to make on each job to be worth while.
There are a lot of factors to take into consideration, distance, storage of equipment, location, do you need to drive or could a cleaner walk to the contract.
Everyone's over heads are different. We take on many small contract of LV - Low Value as there seems to be a fair few about and it soon mounts up.
Our minimum charge on a 5 mile radius is £25.00 + VAT if its 10 miles its £35.00 + VAT and so on.
But there are many other factors to take into consideration and that will depend on how you operate and to whether you are working on Quality or Volume.
All the best
Dave