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MVSNYCguys- these back multipliers on various websites are "guides" not the gospel...take it with a grain of salt...a card is worth as much as bidders are willing to pay and how bad they want it or need it...racking your brain trying to figure out whether it is 10x or 7x or 13x...that is a waste of time...just study the market and relative rarity and you will start to get a gut feeling on how much a particluar rare back will go for...then multiply that by 3... 
a lot of people are starting to get on the rare back kick...i have been into them for over 15 years...