Collateral proportion economy model for the storage Miners

Filecoin white paper mentions (4.1.1): “Storage Miners participate in Filecoin by offering their disk space and serving Put requests. To become Storage Miners, users must pledge their storage by depositing collateral proportional to it.”

My question is, Is there a model for a rough collateral economy? What is the proportion between the mortgage and the storage space promised by the miners? For example, a commitment to provide 1G of storage space requires a proportion of 5Fil.

There will be such a model which will be explicit and well-known. The team is investigating alternatives and I believe not quite ready to make a concrete proposal. It will come soon, though.