Ok first of all thank you ALL so much for your advice on this. Even the ones who say I should be paying 100% of this. I really do appreciate your take on this situation. Let me answer a few of the questions.
EX-DH and his wife knew about the trip because they have visitation hours on Monday and Wednesday evenings plus I don't hide his school activities (or any other activities) from them because I feel it is important that they are involved in his life as well. The permission slips came home on a Monday therefore they saw his backpack before I even got to see the papers. Plus they have requested duplicates of all picture/field trip forms for their own because "I don't share things with them"

even though they know about every activity the child has but rarely show up for any.
What about all of us going on a field trip?? We did that once. It wasn't fun. This woman has ruined birthday parties, she had my fiance arrested right after we started dating (LONG story that the judge literally laughed about), she has lied that my parents tried to run her over with a car. This is NOT a person I want to spend 5 minutes with let alone a week.
As for the money stopping us from taking a family vacation. Last year we had the most amazing vacation (to this very same location) and have actually planned on doing it again this year. The reason the $308 dollars would stop that from happening..........last years vacation TOTAL was $1100. That's food, lodging, gas, souveniers EVERYTHING. Yeah $300 takes a big enough chunk out of that price to stop us from being able to go again. I am NOT made of money and we are looking to have a lot of extra expenses between now and then. My brother is battling leukemia and I am his bone marrow donor. That is a million times more important to me and to my son than this field trip. But the travel alone is going to be expensive. My costs are mostly covered by Be A Match but my fiance and my son need to be there when all of this happens.
As for paying half, I would be more than willing to pay half. I don't have a bit of problem with that. I tried to discuss that very option with my EX the other night but he hung up on me so I couldn't get an answer from him.
I think that was all of the questions. If anyone wants any more details I will be happy to disclose them. I just appreciate the other points of view here.