In my career as a remodeler and builder I was tasked to guide folks through the process of building a new addition or a total remodel of their kitchen or bathroom or maybe build an outbuilding or on occasion to build a new house. This was sometimes a tricky process. But in the end, they had to have some conception of what they needed/wanted before they started. Never once did someone come to me and say, "I have a bunch of 2x4s and 2x6s and a large microlam beam. What can I build with them?" Nor did they ask, "What is the most used material for a second story addition?" I think you understand where I'm headed with this.
I see a lot of new tiers asking all the wrong questions when trying to conceive tying flies or buying materials. What feathers should I buy to tie flies? Should I buy a saddle hackle or a neck? I have a hen skin, what can I tie with it? What is the most important material in fly tying?
When buying materials, you must first have an idea of what you want to tie. Many, if not most materials are specific to a small number of patterns and even then can only be used properly on certain sizes of those patterns. I know you want to get more bang for your buck but there are few materials that can span many patterns regardless of the size. Dubbing is one of those materials but even that is limited by color and texture and other properties. What am I saying? Just this, There are no "one size fits all" answers. You need specific materials for a specific fly. How specific you are willing/able to go will determine how well your flies will turn out. There are some materials that can be substituted for without harming the outcome and we all are happy when "make do" with what we have. But this come with experience and a good understanding of the characteristics of each material.
Know what you want to tie...and then buy the right materials for the job.
My mind is spinning right now and there is too much to say on this subject for a forum. Somebody must have written a book on this because that is what is needed to properly cover the topic. Anyway, I hope you get the idea.