Real vellum is very expensive. It's made one piece at a time from calf skin. There's a modern version called paper vellum that's still expensive, but not like the real thing.
Discussion
I've worked with the paper type vellum, I don't find it good for bookmaking. It's extremely smooth, slightly translucent. I did not find that it folded well, or take pva glue well. I know people who do drafting like it