How should I handle this gift exchange situation?

So, every year I have a Christmas gift exchange party for my girlfriends. We all agreed that $20 would be a reasonable amount to spend on the gift. Everyone that comes knows each other, so we all have an idea of what each other likes. For the past few years there have been a few people that bring a really cheap gift. One accidently left the price on it was only a $5 gift. The person that ended up with it brought a really nice gift. I felt really bad. Everyone is going home with a $20 gift too.

My questions is: What is a nice way to word the invitation stating the gift needs to be $20 without offending anyone? It’s not like they are going home empty handed.