I have an older machine, and bought the darning foot that was available for it. I bought 2 because I had to order them, I can't remember the exact amount, but $15 a piece sounds right. Good thing, after about 6 mos, the metal part, that enables it to 'hop' broke off. I thought, oh no, what if this happens with the second one? So, I just used an open toe foot, no hopping, to see how that would work. I do have a darning spot on the dial that controls the foot tension so used that too.
I noticed absolutely NO difference, and doubt I will even use the second darning foot (I have a button that drops the feed dogs)