Most agencies priorities are : high standards and quick turnaround, i.e. doing it at short notice, and while they have to justify the cost, most charges come out of a tenants deposit or from the landlord, they are more concerned with getting a good job done so they can re-let.
What you need to do is give them a reason just to use you!
My approach is and always has been with these agencies is to offer them something free for themselves, for us its free carpet cleaning for any of there private homes, but could be a proportion of free office cleaning, obviously it has to be justified by the return, we get work nearly every week from the agencies, so a few carpet cleans here and there is well worth it.
Believe me if there is something in it for themeselves they will not go looking elsewhere! Who would ?
This is just something that works for us, obviously you need to tailor it to your own business, but hopefully some food for thought,
Dont work for peanuts, as you need to plan for the long run and have money to run the business and cover the quiet periods.
regards
steve