I have also never liked the whole gift thing at school or at work. It puts pressure on people to contribute even though they might not be able to afford it -- and it often becomes political when not everyone feels the same way about the person who is to receive a gift (as it has in the OP's case).
Any of you reading this who are not caught up in a strong tradition of gift-giving, please do yourself and your colleagues a favor by avoiding it. Instructors are hired to do their jobs and the students should not have to pay additional money to compensate them.
If ... after the semester is over and the grades have been submitted ... you wish to say, "Thank you," write a note or simply smile when you see her and express your gratitute verbally. If an instructor was exceptional, be sure that is reflected in your course eval and any other feedback that might get into her personnel file. She helped your professional career, now you help hers by giving her the positive teaching evaluations that will help her earn a teaching award or a promotion, a raise, etc.
If your school or group already has a tradition of gift-giving, keep it as small and as stress-free as possible. That will minimize the politics and hard feelings that can develop as people don't all feel the same about participating. A small gift certificate ($10 - $25) at the school bookstore is a good idea or from some place like Amazon
.com is a good idea -- or from some other place if you know for a fact that she likes that store. Instructors don't usually make a lot of money, so choose a place that she will definitely appreciate or else a place where she has a wide selection of choices. For example, not everyone drinks coffee or likes going to a spa etc. so only buy those things if you know she frequents those places. Luxury items or stores may be OK for some instructors, but some are really strapped for cash and would appreciate something practical more.