Two things can use "alone"?

Question

One philosophy class and one art history class alone add up to fifteen pounds in books.

And what is the usage of "pounds in sth"?

Answer 1

Without further context we can't be sure whether we are dealing with weight or money, but in either case the speaker is implying that they study several subjects, each of which requires some books, and the cost (I view monetary cost in £ to be the likely meaning) of these books to be troublesome in the context of other expenditure.

Even considering only the books I need for philosophy and art history then I must spend £15. (implied: and there are other courses that need books too, and there are other costs. This is just what I spend on books for two classes.)

So alone here is imply restricting our thoughts to a subset of possible considerations. We consider only philosophy and art history. Philosphy and art history considered in isolation, considered alone.

I infer that this text may date back quite some time, as £15 is a very small sum to spend on academic books these days.

I think it much less like that pounds would be pounds in weight, but if so then a similar meaning could arise.

I have to carry a great deal around the campus. One philosophy class and one art history class alone add up to fifteen pounds in books and then there is my saxophone and my cricket bag that are both even heavier.