Hi Bill, as regards camera price, I'm a bottom-feeder.
I'm not a birder but, last year, I bought a mint Panasonic Lumix DC-G9 for almost a grand when I already had a 50-dollar (used) DMC-G1. With that purchase, I gained:
Dual image stabilization (body and lens) over just the lens.
Live focus peaking.
The ability to better use legacy lenses with help of the in-body stabilization.
Plus, minor stuff, e.g. greatly improved EVF, double the MP, etc.
For me, with my bad eye-sight and shaky hands, the upgrade was worth every penny!
I endorse the popular view that one should have good reasons for upgrading. For example, if one decides to print much larger after only printing A4, then that would be a good enough reason to upgrade to more MP, I reckon.