When I moved in with Mr. R. our incomes were so different that 50% would have crippled me financially.
He already owned a paid for house so there was no need for rent or mortgage payments to be split, instead, we agreed me no longer paying rent gave me the extra income I needed to deal with my finances at least until I was finished with school/tuition.
Between that and receiving a grant I had no student loan debt.
We kept our finances separate although I took over our cell phone bill, reduced our grocery budget by half, took over all the cooking and cleaning and handled a big interior renovation project single-handedly, not to mention painted the house and supervised a roofing project, overhang replacement and replacing rotten beams etc, established a garden (veggies and herbs) and landscaped the property to enjoy al fresco dining and our weekend brunches along with creating a dedicated entertainment area for the seasonal football parties with his friends.
In other words, I might not have had money at the time, but I had skills that changed his life and his place for the better.
He has never paid any of my bills nor I his, but as far as furniture or vacation, we discuss it and sometimes one of us pays all, sometimes we split 50-50, sometimes one of us only contributes X, depending on what else is going on with our budget. If I take a two-month vacation to visit family and friends in Europe that is all on me, if we go out to eat he generally pays, if we need new linens, new coffeemaker or computer I pay.
He has never had a budget but is very frugal - I would be lost without a budget and a plan, it all works well for us - we've never once had an issue.
There are as many ways to split costs or not as there are people. Fair, in my opinion, is a pro-rata split of all expenses based on your incomes. If you come into the relationship with debts and obligations those debts and obligations remain yours alone.
Twenty years later my income is equal to his and we are both significantly better off for combining our assets and skills. It also means we will have a comfortable retirement together. He can afford to retire earlier than expected ...
Good Luck:).