Not that much less than a new one, and surprisinly you rarely see a used KL4 for sale by itself. Your best bet is to probably buy a used KL4 for around $100 - $110 (which is a good deal) and then sell the body and tailcap. This would drop the price of the head down into the $60 to $80 range depending on the specifics. You can sometimes find new KL4 heads for around $85 also. Hope this helps.
-Mike