It's not really your concern what other clients pay to use the trainer's horses or services. But if the "deal" she offered you is not palatable (ie, you feel you are subsidizing the other riders' expenses) then your DH is correct.
Personally, I wouldn't go under those circumstances. However, if I *really* wanted to go, I might tell the trainer that I totally understood her situation, and ask her to recommend who you could show with if you go solo. A good professional will have enough contacts to send a client to school and show with another trainer if they decide not to attend a particular event.