That's true - I hadn't accounted for rare commodities.
But I recall that when I wanted to make Death Touch to deal with monster spawners, the biggest gate was that it was literally the last recipe the Ilari offered me, which meant I had to collect rather a lot of books. After that, grabbing the walnut or cherry or whatever was trivial by comparison.
And although I wasn't necessarily married to the fashion in which it was gated - running about after libraries to find books to randomly unlock new spells - I loved how awesome the spell itself felt, simply because it had been hard to obtain.