It's $40 for a three year membership. If cost is an issue I'm not sure how you'll manage application fees. That said, it is worth every penny to avoid the stress of making sure your letter writers submit their letters on time.
I already submitted all of my stuff, I was just considering interfolio in case I ever have to reapply...and I know that you shouldn't reuse LORs, but it was extremely difficult to find a non-science LOR