I think every couple works out a groove for that — but in my opinion, counting and keeping track of things like groceries is trivial and nit-picky. Just trade off.. Large expenses can be discussed…Generally I feel if one person makes a lot more than the other, then either 1. it can be proportional or 2. type of housing/costs should be considered if splitting 50/50. The idea, I think, is to not let money run the relationship, as most divorces are over this. It also doesn’t mean to ignore it. It seems if both people are financially independent in that they can support themselves, a lot of problems are overcome. But this all gets more complex with marriage and children.
My advice is to split it proportional to income. So if both are making the same money, sure split it 50/50, otherwise draw it along income lines. But then again, what works for a couple is completely unique and complex. 🙂