I was thinking about that. What about an asterisk? Then you could make the tooltip the asterisk expansion. On the bottom of the page, you could also give the expanded version.
This might be the best of both worlds, because you can keep the wording but you also let people know there's more information about what it means.
EDIT: Another idea...
So, you could make the price plus the "x left" be a single graphical unit, thus strongly tying the "x left" to the price, rather than to the seat selection. Then, right below that in a light font, you could say something like "12 left from $y to $z".
This might be the best of both worlds, because you can keep the wording but you also let people know there's more information about what it means.
EDIT: Another idea...
So, you could make the price plus the "x left" be a single graphical unit, thus strongly tying the "x left" to the price, rather than to the seat selection. Then, right below that in a light font, you could say something like "12 left from $y to $z".
So, as an ASCII art example: http://pastebin.com/tS8MPeXK