It definately depends on which of each you are talking about. I say:
Shiny: Worth only like $15 (Shiny Duskull)
Secret Rare: The Charizard goes for like $35 on ebay
LVX: Depends, but Leafeon goes for $40 or more on ebay.
But, SR in general is the most valuable, and the rarest in the charmander line's case.