The description of premium claims in the documentation is to illustrate the potential for a global marketplace enabled by Sovrin. Allowing claims issuers to set a price for a claim could create some very enticing market dynamics.
The principle is that "relying party pays". The relying party is likely to be the one who is looking to benefit from the relationship with the identity owner, so may well be likely to be willing to pay for high quality and potentially complex verifiable data. Of course, if the issuer sets the price too high then nobody will buy.
One interesting evolution of this is that the identity owner can issue their own claims which they may want to charge relying parties to use. For example, it might be that an energy company is willing to pay me to know when my existing energy contract is ending, or a car dealer to pay me to know that I want to buy a car, have the money, and live in a particular place. This "owner issuance" could be fundamental in encouraging people to be in control of their own data.
We still need to design the mechanics (Technical Governance Board) and the principles of operation (Board of Trustees & Trust Framework). There is an interesting clearing & settlement challenge (who owes who what), but we just happen to have a very cool ledger with identifiers, keys and transaction proofs which might come in handy for that!