I'm not going to go into whether the Lebron card is actually worth 300+K too much. Like most everyone else, I think the card went much too high. I think the somewhat analogous comparison would be how much a complete Lebron James rookie game used uniform that he signed would go for. It seems reasonable to assume that the signed rookie uniform should go substantially more than the card.
Back to the card. It's not just any signed Lebron James card. It is a signed ROOKIE card. I don't want to get into a debate on why rookie cards should be much more valuable than other cards for a player. Let's just say that's the reality. (And I'm not going to go into those cases either where the card manufacturer issues the rookie card YEARS after the player's rookie reason. Not going there. Also not going into those cases where the jersey on the card wasn't even that player's jersey or wasn't truly game worn.)
I don't collect modern cards or modern rookie cards. However, from what I can tell, in a player's rookie year, there are tons of different cards that are released. Therefore rookie card collectors look for that player's "best" rookie card. (You can often try googling this: What is
best rookie card?) Typically, these are the characteristics that go into the player's best rookie card:
(1) Manufactured scarcity: the lower the print # the better (I know ppl hate this, but that's the way it is.)
(2) Is there an auto on the card? (Extra points if the player signed on the card rather than on a sticker that is put on the card.)
(3) Is there a game used patch on the card? Extra points if the patch is multi-color/jumbo-size, has laundry tag or logoman, etc.
Cards issued by a major manufacturer (e.g., Topps/Bowman, etc) are more valued than from a minor vendor. TPG grade and eye appeal also play a smaller part.
Therefore, this card has some of the top characteristics that say that this is probably one of the top LBG rookie cards if not the top one. Again, whether this means the card is worth 300+K is another story ...