The cheapest set is to skip them completely. If you're putting a scope on you don't need flip up iron sights.
But, if you actually plan to use them regularly then spend some money and get something good. I find the Magpul and Midwest ones are good enough for their intended purpose which is emergency use but if I was going to use them I would want something better.
If I was planning to use them regularly I'd buy a detachable carry handle with a nice fixed front post setup. Take the 2 min to swap them when I felt like shooting irons.
If you just want them for in case your optic fails my solution is to buy better optics and also forget the mall ninja crap. No one is shooting back at you, a battery going dead isn't going to get you killed, and if you compete in one of the action shooting sports you're going to lose that round because you're not going to pull you optic, flip up your irons and continue the stage.
I have 3 sets of Magpul and a couple sets of the Midwest Ind ones. All but one sit in my parts bin and the ones I have on a rifle never get used. Optics don't fail and my aim is way more precise with optics so I have no use for them, I don't see any reason to pull them off since they're not getting in the way of anything.
Optics or irons, spend the money and get quality stuff, if not you'll always wonder when it's going to fail and it will usually do it at the worst time.