As someone who has bought a fair number of smartwatches and fitness trackers and always over-researches every decision I make:

• See the rest of the replies for info on cheap smart watches. They’re basically a cereal box toy.
• Depending on what you need, the MiBand or Amazfit bands had excellent battery life and there used to be 3rd party apps for your phone that did a much better job collecting and displaying your stats than Zepp or MiFit (the official apps) did. I miss my 1.5 month battery life. Its also possible to use gadget bridge so it’s all 100% offline though I understand its still a bit more rudimentary than a corporate cloud-based solution. I remember the bands I got from them running $25-50 USD
• Used Garmin devices or previous gen garmin devices can be had MUCH cheaper than list price on Amazon or so. I picked up a Fenix 6 a few years ago for less than half of the $600 list price. I love the lack of touchscreen because the button navigation is absurdly fast and no mistouches! This suits how I use a watch much better than trying to put a tiny a 2x2cm touchscreen on my wrist. These are fitness watches, but some have a few smart features. Depends on what you plan to use it for I guess.
• If you are a nerd (a good thing) and want to contribute to a cool project, Pine Computers, which makes the pinebook, pinetab, pinephone, etc. makes a device called the pinetime that is basically a smartwatch that is open to the community’s hacks and modifications. I haven’t bought one because my biking depends on my Garmin stats, but I am tempted to grab one to mess with it.

None of these are fancy “smarts first” watches like an Apple Watch or an Android Watch. I found I needed less smarts than I thought as I usually carry my phone at all times anyway. It is nice to have the doorbell ring on my wrist and to reply to texts by choosing from a few pre-written responses while biking, or otherwise unavailable to text. If you really want a bunch of apps and integration with your phones OS, Apple and Android are the big two and its not really feasible to go 3rd party for the same experience.