So and so. The projects I like a lot and heavily use I give 5€ a month, but there are very few. That would also be the usual member fee for instance for the Wikimedia foundation. Beyond that, it’s usually between a coffee and 20€, depending on how much potential I see, how much it has helped me, and how much the project needs the money. Maybe needless to say: Bug reports help a lot so I place them where I find one. Finally, when it comes to code contributions I find it hard. Usually that requires a lot of preparation and time to get into the depth of the project beyond the API/fronted. And that is, more often than not, a road block for me.
I think though, that the amount given is not a good measure on its own. It depends a lot on what one has and is able to give. There are so many people out there who earn way more than they need (including me, although I am nowhere rich). They could easily give a lot more and cover for a general user base. And the latter, making the project more popular and maybe contributing some bug reports is just as essential.
I’m involved scientific programming tasks (climate models) and we are largely using a mix of prototyping, feature-driven, and big bang models. It’s a result of the requirements for our work. Important is, that our “user base” is extremely small and in the beginning of a project mostly ourselves. The required features are given by the scientific questions and timelines dictated by project timelines from soft funding. Iterations are thus mostly more like “that didn’t quite work, I need another method” kind of arguments. Hence, the implementation of modular and fast evolving design is important and often we try to build our software such that fast development can be done on individual models.
Sometimes theres an idea on how to solve a problem more efficiently or with better performance. And then it’s all about coming up with any proof of concept.
Finally, there is not really anything like a management in our line of work. The professor is usually satisfied if the results work out and papers go out quickly. The rest ist largely up to the group of devs.