I purchased a relatively cheap one from Amazon a couple of months ago. Less than $50, I believe.
I tried some simple stuff when it arrived. I believe I was able to get it to swing within the 94-99% range by holding my breath and by deliberate, deep breathing, over the span of just a minute or two. It definitely was not pegged at 99%. The changes were surprisingly fast. My wife was able to get the same response.
If it matters, I'm at about 500 feet above sea level (not high-altitude at all).
1) don't know how much I trust the stuff from Amazon - see my earlier comment above about working at a pulse oximiter company and all the calibration tests we did.
2) while being an intern and sitting alone in my cube, I had dozen of test meters just sitting there and when I got bored I'd do the same thing and try to get it as low as I could by holding my breath and then snap back up. Our office is at 5000 feet so 95% is normal for healthy folks, I could get down to 85 or so and then hyperventilate back to 97% or so pretty quickly.
I bought one a few days ago (weird coincidence), from Amazon for around $35 at the advice of a friend. Skeptical also, my 97% level upped to 98-99 with deep breathing, and then I held my breath as long as I could possibly do it and it dropped to about 93-94.
That's obviously not a confirmation that it's calibrated or trustworthy, but at least it shows a relative measure that seems real in both directions of oxygen saturdation. Surely if I have a drop due to illness I should notice, even if the number isn't exact.
I tried some simple stuff when it arrived. I believe I was able to get it to swing within the 94-99% range by holding my breath and by deliberate, deep breathing, over the span of just a minute or two. It definitely was not pegged at 99%. The changes were surprisingly fast. My wife was able to get the same response.
If it matters, I'm at about 500 feet above sea level (not high-altitude at all).