Worldwide Distributors via Amazon offers the Tomee USB Super Nintendo Controller
with free shipping
. That's the lowest total price we could find by $5. It's compatible with Windows 98 SE or later, or Mac OS X 10.0 or later.
Of note, Amazon offers its Amazon Prime members the Gtron Classic USB NES Controller for PC for $11.71 with free shipping. (Non-members can spend $25 or more to get free shipping; otherwise shipping adds $4.98.) That's the lowest total price we could find by $9. It's compatible with Windows 98, Millenium Edition, Windows 2000/2003, and Windows XP.
Amazon Prime membership is $79 per year; however, Prime is free for college students for one year, and free for parents and caregivers for three months.