Just how much does condition affect value?

[Exhibitman]

I collect almost exclusively in low grade.

There isn't really a hard and fast rule you can make for how they price out. If you are referring to a mainstream card that can be readily located in a variety of conditions then the cards are virtually worthless except for the superstars, who can be had for less than 10% of the price of a near mint version if you are willing to wait a bit. I try to pay even less, just to make it interesting. Like a beater T206 red bg Cobb. I waited for quite a while until I found one for around $100 less than 'market' and bought it. 'Junk' boxes at the National can yield even better deals. Also depends on how FUBARed you are willing to get with a card.

There is no rule on pricing true rarities. It is strictly what the buyer will pay. I pick a price I would like to get for a card like that and tell prospective buyers who want to dicker about it if you can find the card somewhere else, buy it.