Quoting Blair Waldorf (SuperKink):" Oh no this money was due by the 24th and the field trip isn't until tomorrow, so a week ahead of time. ... [snip!] ... period. End of sentence. They do not refund money. I thought that was ridiculous. I'll definitely be sending them the letter."
Normally field trip policy is laid out in the handbook, so check there to see if it's stated. But honestly, if he got himself suspended, then no, I don't think a refund is in order, and you would honestly be wasting your time writing that letter. He misbehaved, he is having to suffer the consequences, and unfortunately, you have to eat this cost.