WoW API: TaxiNodeCost
Returns the cost, in copper, of the given taxi node slot. Returns an error "invalid taxi node slot" if no taxi map is up or an invalid slot is used. Returns 0 if the current node (the green "you are here" node) is used or the flight is free.
For some reason, this returns the original cost of the given slot, not taking into account any reductions you get due to faction.