The PP156 was not a limited edition - they made as many as they could sell.
There is no price guide, I'm afraid -they tend to go out of date quickly, anyway. As you probably know, many of the 'run of the mill' paperweights have dropped in price over the past few years, but the choice pieces (good antiques especially) continue to climb. I doubt that any modern pieces go up much in value until the factory /maker closes, and new pieces are no longer coming onto the market. Thus Perthshire weights seem to have held their own over the last year or two. That said, I have seen some pretty heroic asking prices recently - which are very different from sale prices, of course!