There is a din by sheviis that if a person sells peiros sheviis, the kedusha transfers to the money and the money has to be used to buy food and eat that food with kedushas sheviis. Pnei Yehoshua has an interesting chakira that I have to preface with a story. A long time ago when my kids were little we went to the Brooklyn Children's Museum (a wonderful place) either on chol hamoed Sukkos or around that time. The BCM is located right near Crown Heights, home of Chabad, and that afternoon the museum had a little talk given by a chossid who came to tell the kids about esrogim. So this Lubavitcher chossid stands up in front of an auditorium filled with kids of all colors and races, takes out an esrog, and asks who can guess how much it costs. One young non-Jewish boy started off the guessing at something like 79 cents. Was he in for a surprise! But the truth is that if it wasn't sukkos, you wouldn't spend more than 79 cents for an esrog, if that much. The Pnei Yehoshua asks: if you spend $100 on an esrog which is peiros sheviis, does $100 have kedushas sheviis, or does only $.79 have kedushas sheviis, because that is the value of the fruit if not for the mitzvah-value inflating the price?
The gemara (Sukkah 39) writes that during shemita year one cannot buy an esrog because the $ given to the am ha'artz farmer would have kedushas sheviis and one cannot give more than the value of 3 meals of kedushas sheviis $ to an am ha'aretz (see Rashi and Tos as to why). Tos on the spot quotes other gemaras that indicate that an esrog is not worth so much $, certainly not more than the value of 3 meals, so what is the gemara talking about? Answers Tos that those other gemaras are the $.79 esrogim you buy all year, but on sukkos, everybody knows you pay a premium for the mitzvah:
התם באתרוג פסול דלא בעי ליה אלא לאכילה אבל הכא כשר והדר לברכה דמיו יקרים:
You see from Tos that the entire value of the esrog on sukkos gets the status of kedushas sheviis, not just the $.79
It could be what's driving the PnY's chakira is not simply whether you measure value based on a one time a year price or the normal price, but more fundamentally whether kedushas sheviis is transfer of the value of the fruit, or whether it is the inverse, if you will, of the mitzvas achilas peiros sheviis. In other words, since you failed to eat the peiros sheviis, you "owe" that makeup value. If the latter is true, then the value of an esrog is simply its value as an edible commodity, not its true market value.