I would say, it depends. But in co-writing situations, I always do 50/50 without taking an extra piece of the cake for production and mixing.
I am lucky to say, that my co-writers until today (and I haven't done much yet) were all wonderful people. They were doing a lot of other work, like flying to LA to present the song to music supervisors... (I live in Europe, so they are worth gold to me). In my opinion, when two people give their very best in every way to make things happen, I don't ask for an extra percentage for producing and mixing. Simply all 50 / 50.
Have a great time!