Would magi try to learn origami as a way to make cheap magic items? If so what items w/could be made using origami?
Origami? Paper? Cheap? In Medieval Europe?
Or are you playing in a modern world?
There is no paper to make origami with. There is parchment, but that's basically very thin leather, and not cheap.
The least expensive materials to make anything with are probably stone, clay, cloth, or wood, if you want them to last.
There is paper in Europe in the 13th century, certainly France had a paper mill before 1200. However, medieval paper is entirely unsuited for origami and should not be thought of being similar to modern paper, rather think of it as a type of cloth.
So, there is no paper suitable to make origami with.