I can't tell you what you would have to pay but it should be in reasonable proportion to what you'd stand to win. You need a CA trusts and estates lawyer to help you. Base on what you've said your father has misappropriated the trust funds but as I haven't seen the trust document I can't be completely sure. Get a competent lawyer.
Generally, contingency fees range from 1/3 to 1/2, depending on the complexity of the case and the costs to advance. From what you described, it appears there was a breach of trust if your father is the trustee and you and your siblings are the named beneficiaries. However, this all depends on what the trust agreement says. I handle trust litigation and would be happy to consult with you further on this offline.