The Laphroaig 40 YO and the Laphroaig 17 YO single cask bottled for the Feis Ile 2003(?) were amongst the finest Laphroaig that I have ever tasted. The Laphroaig 30 and 40 YO are quite different, with the sherry most present in the 30 YO. On the other hand, the 40 YO was more complex, without being tannic. Personally, I prefered the 40 to the 30 YO Laphroaig.
About the price difference? This is again very subjective. For the price of 1 x 40 YO, you can buy 3-4 30 YO. Personally, I would almost prefer to buy a few bottles of the old 15 YO with the red letters
Up to you