From Wiki]
A Time and Labor Intensive Harvest
It takes anywhere from 15 to 25 years for the trees to begin producing the seeds and up to triple that time for them to reach top production. The majority of the North American harvest comes from wild, uncultivated trees. For the most part, the seeds are harvested by hand, a contributing factor to their expensive price tag.
The pine seeds are found in the pine cones and take about 18 months to mature. Since the pine nuts are ready to harvest 10 days or so before the cone begins to open, they are very difficult to remove. To speed up and ease the process, the cones are placed in a burlap bag and left in the sun to dry for 20 days. Next, the cones are smashed, releasing the seeds, which are then separated from the cone by hand. This is another very time-consuming and patient-testing task.
The pine cone is not the only covering for the seed; each pine nut has a second shell that must be removed before eating. Some of these shells are thin and easy to take off whereas others are thicker and more challenging. All of the above factors contribute to the understandably high price of the pine nut.
no wonder they're so expensive.
I've used them in the past but other kinds of nuts can be substuted with no loss of texture or flavour.