All very valid points so far. Again, I am not referring to dealers who are trying to sell vg cards for NM book $'s...I think we can all agree those days are over and those dealers are done for in large part to eBay, and thankfully so.
But a couple years ago I was on the other side of the spectrum and attributed a card's "value" to finalized eBay sale prices ONLY. Now, after some seasoning, and checking out cards at various venues (flea markets, cards shows, shops, other websites) my idea about a card's (monetary) value has evolved--unfortunately in a slight upward direction
While I wouldn't pay $600 for a '58 Mantle, if a dealer had one at a show for $250 vs. the $200 ones on eBay I would CONSIDER paying the extra $50 if I thought it was strong for the grade, etc.