Its an age old battle between Supply and Demand and the cleaning business has always had to deal with low margins, high costs (employees wages) and increasing compliance.
Some companies choose to subcontract any work they secure to other companies in the hope of paying those staff less for the same work, some companies pay their staff in cash to circumvent payroll and company taxes.
Customers and Suppliers both need to be accountable for their part in this balancing act. If you, as a customer, are pushing for the cheapest rate then you may be indirectly contributing to the problem.
To combat this; Know the award rates that compliant companies must pay staff, ask for a breakdown of costs to include wages and penalties, overheads, consumables, equipment, supervision, government costs (such as portable long service leave) and profit.
This will give you a good idea of where the costs stem from and allow you to ask more questions.