You can use the end of your stethoscope or even your hand as a reflex hammer (really only works on the patellar though), and you can get free ones at residency fairs or from drug reps if they have them. Or just borrow. And you can usually snag a Sanford from a drug rep--although not always--and this is something you will want to have.
If you have a good library at your school, you can usually check out test prep books for a week or so (longer if they are older editions).
You can also get together with a few classmates who are on opposite schedules and each one of you buy the books for one rotation, then swap out after the shelf. That worked really well for me.
Must haves (things you won't want to borrow):
Tarascon pharmacopaeia (if you don't have a PDA)
Boards and Wards
Maxwells
Sanford
Johns Hopkins IM (little blue book)
A pregnancy wheel (free from reps)
That can probably be done in <$80, especially if you get the books used.