I have an Rx 590 that gets around 500k/day, if it can be kept busy. Lately I've been having a problem with the work fetch hanging and the card going idle until I restart the client but that's a different problem. Anyway I paid around $150 for that card, I think that's pretty good value. I haven't checked lately but when I first started the newer Navi cards weren't reported on the Folding forum as being very productive. That's probably due to driver problems, AMD has a pretty good record of improving that over time but it does take awhile.
Nvidia, I'd look at the 2060 series, or maybe even 2070/80 as the Super cards have pushed those prices down a bit. If you're concerned about power the 1660 series will be hard to beat. There's supposed to be a CUDA client coming out, it should be more productive for Nvidia cards than the current app.
You can also check the used market. There's not much of a local market for good gear in my area. I've had very good success buying used hardware on Ebay.
If you want a card to also support BOINC and want to run every BOINC project, remember GPUGrid is Nvidia-only.
