Leaf is notorious for its blank backs, but I wouldn't spend that kind of money on one without a slab. There are too many really nice looking counterfeits (or 'reproductions' as the first sellers call them) on eBay in the Leaf world. Run a search for any marquee Leaf card and the repros come floating up.
As for grading, SGC has been inconsistent in its treatment of blank backs, even in the same order:
This was in Sports Collectors Daily:
I also question the price of the card assuming it is genuine. I have been collecting print freaks for many years. The only issue I've ever seen that
really benefits financially from a major printing blow is T206, and I am not sure that a blank backed Plank would command a premium over a comparable complete one. There are a number of specialty collectors who'd pay extra for print freaks of players they happen to collect but that is a pretty small group, especially when you are talking elite pricing to begin with. I might drop a few bucks on a blank back specimen as an ancillary item for my main collection (as I did with the Aaron) but not the sort of money the seller wants. His other blank backs were overpriced by a factor of 10 IMO.
As for which cards might be found blank backed, it is entirely a happenstance event due to a missed printing pass on a full sheet. Whatever is on the sheet will end up blanked. We know Leaf used 49-card sheets, so there should be 49 blank backed cards out there from the sheet that yielded the Paige and probably the others the seller has.
Anyhooo, that's my $0.02 worth from the freaks and geeks collecting world...which is about the right valuation...