SP's and v4 block assignments

Hello, Looking for information on the current standard practices for charging customers for larger than default v4 assignments. Especially with the rapidly depleting v4 space, how are SP's handling these requests? Is it safe to assume customers requesting larger blocks are willing to pay a premium? How much are SP's charging and what are the thresholds? What are default allocations based on? (ie: size of the circuit, type of product, etc...) Are SP's requiring more strict justification for said assignments? Thanks in advance, -drew