First you'd have to find someone who has a yearbook signed by both of the boys and who is willing to sell it to you. You're not going to find a copy of the 1999 yearbook signed by both of the boys because the yearbooks hadn't come out yet at the time of the shooting. So it'd have to be a yearbook from 1998 or prior. And if someone has had it all this time and are just now wanting to sell it, it's probably because they are having financial difficulties and really need the money. So I'd imagine they are not going to let it go for cheap. If I had to guess, I'd say (at the very least) a couple thousand US dollars.
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“And may you grow to be proud
Dignified and true
And do unto others
As you'd have done to you”