There's a very wide range in cork prices.
Did you check out Bob Block's book yet? Does it have updated prices? In his older edition he would give prices for basic corks in different categories (such as prizename and popeye, if I recall correctly). And then he'd give multipliers for when they had special features. Like it might be worth 2 times as much or 1.5 times as much with a certain feature. I once used the multipliers in his older edition -- used about three at once which I thought applied to the same marble -- and ended up with an estimated value of about $2500. I eventually sold the marble for $80 to someone I trusted to tell me straight what the value was. I wasn't sure about my grading at the time, and so I asked about that and about what he really thought the marble was worth. It was a wet mint marble (which I had been afraid to say in my description) and he said he would have paid $100 for it if he had a chance to view it in hand at a show.
My point?
Ummm ... not sure .... oh yeah ... there's really not a simple formula -- there's not even a standard corkscrew. The most common corkscrews probably sell for less than a dollar. Others sell for hundreds.