You might get a little more polish in a more expensive stirrup, but I suspect you are mostly paying for name. Some expensive stirrups have features that may be worth paying for, depending on the person. I wouldn't want to use magnets to hold the stirrup to my feet, but someone doing a sport may be willing to pay for that little bit of extra security. Stirrups with hinges or springs do nothing for me, but may help someone else.
With western stirrups, I think you can get a lot more out of a more expensive stirrup. If a stirrup is made of wood, then the quality and drying process used is pretty important, or the leather wrapping may be higher quality. The stirrups on my Circle Y are much higher quality than the cheap western ones I used to use on my Aussie saddle. And a stirrup like this: EZ Ride Leather Caged Stirrups
May work better for some folks, and I can see how quality may vary with price.