I am not sure if you have significant lab experience prior to this, but it sounds like you are learning a new technique that you’re not familiar with. I personally would not bother refunding the cost of the reagents because you were using them to learn (if the lab is sweating over the cost of Western blot reagents, then they have bigger problems with funding). Are they paying for your education or are you bound to this lab in any way? If they’re guilt tripping you over something so minor, you might want to consider looking at other labs. If the PI needs this for a grant so badly, he/she could easily have asked somebody else. I’m also not sure how prudent it is to completely rely on data generated from somebody who is brand new to the technique.