It really depends on if it is once off crew or regular.
For once off, if I invite you, I do not expect you to contribute anything. I will send you a list of what is recommended to wear and what to bring to eat / drink if you don't know. Everything else is provided - even the seasick tablets. If you choose to offer a contribution, then that is always welcome - but when you see me put in 100 litres, please keep the 50 bucks in your pocket....
If you want to hire me to take you out, then I will respectfully decline and suggest you go with a professional charter. There are many legal issues and pitfalls here, and having done it for a few years, I have no wish to compete with the genuine, hard working charter skippers.
When it comes to regular crew, we just take turns with the fuel. Over the years it evens out between inshore / offshore trips.
The fish generally goes with the angler or we take a loin each on a bigger fish and when we have enough we release.
I know it sounds harsh but on a small boat, it is best for you to advise the guest on what to bring. It can be challenging to store tuna rods, tail rods, tackle bags, lunch bags etc for 4 people on a 5m boat.