Listen man, I've come across a few of your topics and haven't commented on them because people like
@Metalcat are wiser and better at explaining 'life stuff' like this. I'm commenting because I've struggled with being a 'Nice guy' and having validation issues for practically my entire life that I am just now coming around to. My life is becoming infinitely better since changing my schema.
I will enjoy it because I obviously enjoy spending time with my GF and I really like her kids so it will be fun.
I would like to help with the trip costs but I am unsure what is fair.
This is cognitive dissonance. You say you'd like to go on the trip because you like spending time with your GF and her kids. Why are you making it transactional? Why are you fabricating a covert contract? If you contribute too little towards the trip, you're worried that your GF's perception of you will falter. If you give too much, you feel like you're owed something in return (i.e. praise, sex). There is no 'right answer'. The outcome is irrelevant. The problem is not what is fair.
The problem is your sense of worth & validation is tied to what others think of you. You're on an anonymous internet forum posing relationship questions to seek validation.
Dude, no one actually gives a shit what is 'fair'. It's YOUR relationship. You're seeking validation from external sources under the guise of 'fairness.' You are repeatedly falling into an ego trap. STOP IT.
You want an answer for your hypothetical scenario?1. Delete this topic. Stop looking towards external sources to glean information on what is fair or not.
2. Decide what YOU want to contribute to the trip, anywhere from $0 to the entire cost. If you don't want to go on the trip in the first place, say it and own it. Don't go on the trip while covertly expecting something in return. Be honest with yourself.
3. Tell your GF "I want to contribute X to this trip" and leave it at that. Better yet, don't even say it. Just send her the money (or don't) and shut up. Do not open a long, drawn-out discussion on why or what is fair.
4. Own your decision and do not covertly look to get something out of it. You are a grown man. It doesn't matter if you're praised for your decision. It doesn't matter if you're disapproved of either. People in life are going to take you for granted all the time or invalidate you regardless of what you do. These outcomes do not change your fundamental worth as a person.
5. Respect yourself more as a result of this process and become a stronger, more solid person.