I know any commodity is only worth whatever someone is willing to pay. (Economics 101).
Keeping this in mind is there any general rule of thumb when it comes to equating book value to real value when it comes to sports cards?
Econ 101, supply and demand. At any point where demand is lowered after a steady supply you will see a huge drop in actual value while book value may not really adjust. Think of all the product in 1988-1990 where even 10 years ago you had people clamouring for the rookies of Sosa, Alomar, Biggio, Thomas, Williams, Glavine, Smoltz, Johnson, and Griffey, now you can probably find them in quarter boxes at major shows. You take current cards of similar book value from a popular set, say Bowman Prospects, you'll find people more likely to hoard them rather than trade, and the actualized price when sold is much higher percentage than some of the older stuff.
It depends on the era the cards came from as well as the types of cards
I wouldn't pay even 10% BV for decent stuff from the 80s early 90s...I wouldn't pay anything for a base common from that era...so 0%?
Base Cards of newer stuff are practically giveaways too. Some card shops are generous IMO by paying $20 for a 3200 box of base cards (stars, semi-stars and commons)...the cards when valued individually probably total $1000-2000 BV....so SV in this case would be 1-2% of BV.
On the flip side, many would pay 50-100% BV for certain high-end Autos and GUs of some top Rookies and Stars they collection. Sometimes even, a red-hot name/card will sell for many times more what it books.
It really depends on the player in the card. When the player is hot, then his actual value goes up and vice versa. There's really no true way to compare book and actual value. I look at both values when I trade. If you have been trading for a while, then your gut should tell you whether or not the trade is fair.