Although I’ve never used it I heard the Davis Mark III plastic sextant is meant to be decent, at only $59.99 it’s pretty cheap.
It’s a good starter sextant to mess about with.
Found this review of it online:
“I expected the Davis Mark III to be merely a teaching “toy”, but to my pleasant surprise I’ve noted how effective this very simple sextant could be as a get-out-of-jail-card should everything electrical fail during an ocean passage (as happened to me a few years ago after a lightning strike). This plastic sextant is very light, will not corrode, and seems unlikely to degrade in prolonged storage on board. The instructions are a useful aide memoir too. It is simple to use (rather than finely adjusting the arm to the sun, it’s easier to set the arm very close to the relevant angle and then let the celestial object gently come to the horizon). There is no drum type of adjustment, simply an (at first sight) rather primitive vernier scale, calibrated to the nearest 2 minutes of arc, but in practice it turns out to be remarkably consistent, accurate, very effective and easy to read. I have three minor provisos though. 1. One is that there is no decent storage box with it (but a simple plastic snap-top with sponge in it would do). 2. Another is that there are rather too many internal reflections in the sight tube for my taste, but matt black paint and a thinner but longer plastic tube pushed inside it improved it quite a bit. 3. The third is that with the slightest of nudges the index mirror loses its alignment (but this seems to apply to all the plastic Davis sextants I’ve met). It is not a significant problem because even though there is surprisingly no adjustment at all on the horizon mirror, adjustment to the index mirror is very quick and simple and effective because of the two knurled screws … no tools required. Besides, I have always taught that it is good practice to do a very simple check on the alignment both before and after taking a series of sights anyway. In summary, I consider this machine to be good value for money both as a backup at sea and as a practice sextant. Too often skippers have a wonderful and very expensive sextant which is quite frankly far too valuable to use on a bouncing yacht, except on very rare occasions … the result is that they tend to be carefully packed away somewhere inaccessible below. I believe this little plastic sextant would be used a great deal more often, and it is quite accurate enough to be genuinely practical.”