[inquisitor]

The rates at which MVNOs buy airtime from MNOs cannot be that much discounted for the following simple reason: Regulatory authorities set mobile termination rates based on an assessment of Capex and Opex involved in the deployment and operation of mobile networks. The principle is that termination rates should cover the costs of building and maintaining mobile networks plus financing costs and a small profit. The highest costs of a mobile network lie in the radio access network (basicly the towers and all the fibre and microwave links to the core network), so the core network components which a full MVNO has to build by himself make up a relatively small part of the total costs.
Now if an MNO sold airtime at rates too much below the MTR they would prompt the regulator to step in and lower the MTR as this would imply the costs of the RAN are lower than assessed by the regulator before. So MNOs must be very careful when selling airtime to MVNOs as they risk their own MTR to be cut.