Good spotters generally cost a good amount of money.
There isn’t a whole lot to choose from that is $400 or less, but there are a couple of compact options that have their uses.
I did a comparison of small spotters quite a few years ago and I revisit it once in a while. To my surprise, the 16-30x50mm Minox MD I looked at back then, still holds up quite nicely. The only other small spotter in this price range that I likes is Athlon Ares UHD 7.5-22.5×50 that is even tinier and offers 7.5x on the low end for handheld use.
Unfortunately, both of these are available with angled eyepieces only. Straight eyepiece works much better for handheld use.
With larger spotters, I don’t really like any available options and you should really be stepping up a little bit. If you must stick with $400, Vortex Diamondback 16-48×66 is decent.
There are a few other options in this price range, but I have not really seen them. The guys at Athlon are saying that their sub $400 Argos HD 20-60×85 is really a step ahead of the competition so I might look at it. They know that I am picky, so if they are willing to go out on a limb like that they probably have something good on their hands.