most business cost are fixed, it cost the company the same for you to do nothing or a small job....... apart for the cost of the diesel to get there.
I can sit at home and the unit will still cost the same, so will all the insurances
the only time the amount of your min charge makes a difference is if you are busy all the time then if that £25 minumum charge job stops you doing a bigger £80 you lose out......... but if that minimum charge jobs replaces you watching Jeremy Kyle in a morning then once the diesel is paid most of the rest is profit