Is it acceptable for the entire class to pitch in a dollar and get a small, inexpensive gift for a professor who really helped all of us out?
Sounds like a very nice gesture. Go for it!
Yes! My thesis cohort did this.
Yes I have done this before. Doooo it.
We did this this semester! Turned out great. If you can, make it personal to something you discussed/learned in class.
Never did it in undergrad, but perfectly acceptable.