Whoever is going on the trip with him should be the one to pay your son's way. You had already planned on going with your fiance and paying for your son. It sounds like stepmom and Dad got wind of this trip and decided to one-up you by getting there "first" and then trying to make you look bad for not paying for your son. I wouldn't play this game. Tell your ex that if they cannot afford to pay for all three people to go, then stepmom stays home and make this a daddy/son trip. I will say that if adult/parent travel were not an issue and your son was going by himself, that you and his dad should split the cost but that's clearly not the case here. I certainly wouldn't cancel a family vacation over this nor would I allow an 8 year old to make that decision for my family.
-Astrid
Astrid,
Did you sneak into my house and get ahold of my keyboard while I wasn't watching?
Let me say hell would freeze solid before I would fork over one red cent for this trip now that Stepmom had funkified it all to pieces. The OP is right....The cost of the field trip was already HIGH. Considering they had a "parent's price" of $320 (?) it seems as if they were anticipating a decent number of parents to want to go along on what must be a really nice trip.
And the OP and her spouse/fiance had decided to do just that. Spend around $1000 minimum for THEIR family to go on a family vacation, albeit with company. The Stepmom threw a massive monkeywrench into the whole thing by signing paperwork for the son, herself and the father to go on the famioy vacation instead. And from what the OP says, she signed for the kid's expenses.
Let's summarize: Stepmom had diddled OP's family out of their planned vacation, so they will plan another one. But remember, they do not have a money tree in the back yard. Stepmom said she and the dad would pay the boy's expenses, plus their own at $320 each. Now Stepmom and Dad seem to have sufficient funds to pay the $640 for the two of THEM to go on the trip, but no money for junior. They expect the OP to pay for that.
Sooooooo....essentially, they expect the OP to subsidize 1/3 of THEIR family vacation. They screwed the OP out of the exact same family vacation and are nervy enough to want her to pay for a third of THEIR vacation. Well, I'll hand it to Stepmom. She has cajones of steel. At the risk of channeling Miss Whitney Houston, I can only say, "Oh hell to the no!"
If they only can get their grubbies on $640, then remind them that's enough to pay for dad and son. Problem solved. As Astrid said, the one/couple who goes on the trip is the one/couple who pays for the son. OP, save your money for YOUR family vacation and feel no guilt. Don't let Stepmom's attempt to snooker you into subsidizing their vacation work. Don't jump on the guilt train. She tried the tactic and you need to send a message that it will not work or she will pull this crap again and again.
Nip it! Nip it! Nip it!