Is it inappropriate to give one faculty member a larger thank you gift than another?
I would suggest sticking with a card, but writing your appreciation as extensively as you wish.  There's no reason for the contents of the cards to be similar.

Even ignoring the question of differences between faculty members, gifts can be tricky for professors to deal with, ethically, particularly when it's regarding a favor in a class.  

Finally, I guarantee that a heartfelt explanation of how much the help mattered to you will mean a lot more to the professor than a $10 mug.