You have asked a very good question… and one that is perhaps hard to answer to your satisfaction.
The simple answer is, that you can’t equal expensive to good. Or sometimes you can, but it really depends on the whisky in question and some other factors….
There are a lot of very good (younger) whiskies out there, which are fairly inexpensive, Ardbeg 10, Lagavulin 16, Laphroaig 10 & QC, Balvenie 10 & 12, Glenmorangie 10, Dahlwhinnie 15… the list goes on and on.
So clearly one can’t state that an inexpensive whisky is of lesser quality.
What makes a whisky expensive is age (It costs the distillery more to store it, there is greater loss of product due to the angles share and it is often intentionally positioned as a premium product) and rarity. (Supply and demand at work.)
But there are some truly outstanding whiskies in the old/expensive category… and there are a lot that is less outstanding, perhaps merely good.
The best quality/age to price ratio is perhaps found in whiskies in their late teens/early twenties. After that you begin paying for the other factors mentioned above.
Following the discussions at forums like this, can warp ones perspective of price, I think. This is due to the fact that for many of us here, whisky are a hobby (borderline obsession, would perhaps be more correct), and many of us spend more on whiskies, than can be rationally explained I think.
Not so long ago I had the chance to try a dram of the legendary Ardbeg Provenance at a fair (And to pay an arm and a leg for the privilege) Beforehand I also agonized if I should really pay that much money for a dram and I asked around here.
In the end I did buy a dram of this and yes, it was a great whisky and I am glad that I have tried it. But if I look solely at quality, I’m not so sure if I could not have gotten a equally good Ardbeg at a fraction of the cost. On this occasion, I chose to pay for the legend of the Provenance. (And thus earning a small measure of bragging rights at my whisky club.
Personally I think that after a certain point the price of matured whisky becomes too steep. What that point is, is a personal matter, depending on what you can afford and what you are comfortable to pay for the experience.
So to return to your question: Is the HP you have your sights on, really good enough to justify the price…?
Perhaps, but it is really something you have to decide for yourself. If you can afford it, by all means go for it. But I do not think that you should expect it to be twice as good as the more affordable HP18. (I have not tried the Park Avenue HP).
We have discussed this question before at some length:
Age vs. oakiness
Is an older whisky by definition a better whisky?
Edit: fixed couple of typos