I bought a 128GB Google Pixel 1 in “very good” condition on eBay in December 2018 for $170. It replaced a hand-me-down (free) iPhone 3GS from 2009 that still works fine, but which maxxed out at iOS 6 and thus won’t run modern apps.
That was a bit more than I originally intended to spend, but I wanted plenty of storage and no serious damage as I’ll be carrying this phone for years to come. In reality, I’m using only about 30GB of storage even with dozens of apps, but too much space is a good thing. The phone looks nearly new with only a small scratch on one corner. Screen is flawless, etc.
The original Pixel is still getting monthly Android updates as soon as they are released and I upgraded to Android Pi (9) after activating the phone. Performance is still great, though the Pixel 1 camera is not quite as nice as the one on Pixel 2/3. It’s still unbelievably good when compared with my old phone 
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It’s not perfect, by any means, but 6 months ago it was a good value for a phone that I didn’t have to root in order to get the latest operating system. If you don’t mind rooting a cheaper Android, then by all means go for it, but you will probably have problems and some of those problems may be unsolvable.
BTW: Don’t get the original Pixel XL; it had screen problems.
The best “used” values are changing constantly as new phones come out and supplies of “old” phones come in and out of stock at refurbishers. The original Pixel is probably not such a great value today, but I’m mainly pointing out that used phones can be an excellent option if you do some homework and find a trustworthy seller. My next phone will likely be used as well.
The Pixel 3a appears to be a good value for a “new” phone and will probably be far more popular than the Pixel 3.
The OnePlus phones have traditionally been good values. They use near-stock Android and get rapid security updates, but:
# OnePlus 6 went upmarket slightly, but OnePlus 7 jumped upmarket substantially. The company has admitted that these will no longer be mid-tier phones 
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# Older OnePlus phones have maintained insane resell values, making used 4/5/6 models rather poor values.
Though all phones are untrustworthy to some degree, any phone that doesn’t get iOS or Android updates should be considered to be someone else’s; you should not do sensitive or financial activities on such a device. Cheap phones do not receive updates as a rule and even those that do may be updated more than a year after Google releases the patch; too late to help anyone.
I’ll also mention that a security research firm recently tested the 30 most popular “financial” apps from banks, credit card companies, etc and found that all of them, both iOS and Android versions, were insecure and poorly written. Some were worse than others, but none sufficiently followed industry standard security practices. They easily hacked into all of them.
Lastly and as others have noted, if your phone is slow, try removing the crap. My Pixel 1 is fast and smooth in Android 9, unless I add more than a couple of widgets to the screen. Widgets will kill performance like nothing else. If you don’t need an app, remove it. If you don’t use a screen icon or widget, remove it.