"Sunk costs" are those costs you have already paid for an item or service. People in this country have a tendency to pay too much attention to sunk costs as a determinant for future actions. For example, if you bought a ticket to a concert you will likely feel compelled to go to the concert because you paid for it. The better way is to make the decision to go or not go based on what you actually want to do. The money is gone either way.
Yesterday I paid for new lenses for my current eyeglass frames. I paid a lot for these lenses and as soon as I left the store I started to regret it. But it is done. Regret serves no purpose, unless it is to use it to decide how I will approach such purchases in the future.
I have a rule about not buying an item from someone over the phone or at the door the first time that person calls or comes by. That rule has served me well. It gets me out of having to make a decision until I have given it more thought. I simply tell the person, "I have a rule...". That person usually says, "Yes, but if you buy now you'll get this deal. If you put it off you lose the deal." I shrug and say, "Well, that's too bad but that's my rule."
I can now make a rule about such things as these glasses that when the purchase is some amount over what I expect that I will delay the purchase. I may still buy them, but I'll feel better about it. As for this time, I refuse to dwell now on the negative aspects. I have simply created a new rule that will help me in the future and I will enjoy those new lenses.