Um, what's wrong with saying that you're not interested in doing a group gift? You're adults, you get to decide your level of involvement now.
She tells her mom "Hey, that's a nice thought, but I wasn't planning on giving a gift like that and I'm not interested in doing a group gift situation at all, so I am not going in on it, thanks anyway."
If she complains - "so sorry you feel that way, but I'm not interested." and then change the subject and/or end the conversation. (and a nice shut down to go along with this particular situation: "that's not in our budget, so no."
You are allowed to say no. You are allowed to disagree. You are allowed to leave/end conversations you don't want to be in. The key is to do so firmly, gracefully and without getting dragged into defending yourself or making excuses. Just say no (politely but firmly) and move on. It is a skill that is quite difficult to master in regards to family, but it is vital to learn if you have family members that are used to running the show and still expect to maintain the old parent/child dynamics.