It's funny. Based upon my experiences as a half-assed hobby bowyer/selfbowyer, were that same "few fps," and shot feel for that matter, decided by the bow design and crafting itself, versus the string, then the price difference would not be $4, or even $40. Imho one could argue more tlike $400. Kinda ironic.
Once again you turn on a light bulb. I'm often amazed at some very nice (and often very expensive) bows that have a very...basic string on them. Not always but often the string can make a big difference in the feel, and some difference in the performance. Noise can be a huge difference. I wonder how many bows have been sold/traded when all they needed was a decent string on them.