This is one of the most contentious areas of precancel collecting. Precancel collectors have traditionally wanted to pay only the value of the precancel, regardless of the value of the stamp.
While that works for the vast majority of stamps, as most are in the .25 minimum catalogue range, what is the value of a stamp which is worth more than minimum catalogue? The majority of precancel collectors still expect to pay only for the value of a .15 precancel, even if the stamp it is on is valued at $20.00!
I tend to list my precancels in my collection's inventory with the precancel value added to the stamp's value, just for insurance purposes. Do I expect to get that when I sell the item? Probably not, especially for the common stamp/common precancel.
If you are looking to sell the precancels you have, the best advice I can offer is to list them as what they are, accurately describing any faults on the stamp and then start the auction at a low bid amount you could live with, if the stamp sold for that amount.
Here's a few examples of precancels which, because of clarity of the precancel and centering, I would not be willing to accept a minimal bid on. All of these are from my collection.