Just been collecting for 12 years or so now, and finding that provenance is increasingly important to me. The premium I would pay totally depends on the coin. but I'm not interested in acquiring coins from a particular collection (Brooker excepted, a focus at the moment), rather as an assurance of their authenticity. Some coins I would not buy at all without some provenance (Northumberland shillings, Charles I siege issues, wreath crowns, to name a few), and would pay a decent premium for cast-iron provenance (25%?)