I know there is a wide range of prices on the 18 yo. Vermont has it for $50 while PA is about $75. Like anything else you gotta shop around.
Indeed the Highland Park 18 seems to be priced all over the board.
I see it around Houston priced from $55 up to $95.
For current bottlings, Specs Liquors is the place for best prices - they list HP12 at $35, HP15 at $55, HP18 at $85 and HP25 at $283.