I've jotted down some quick napkin math. Hashrate isn't necessarily perfectly linear to total memory bandwidth, but it is a somewhat reasonable analogue to give round numbers.
RTX 3090* ~935-1008 GBps
RTX 3080* 760 GBps
RX 5700 XT - 480 GBps - 58 mh/s (eth)
RTX 2080ti - 616 GBps - 52 mh/s (eth)
GTX 1660 super - 336 GBps - 30 mh/s (eth)
RX 580 - 256 GBps - 30 mh/s (eth)
I think it would be somewhat reasonable to take a guess at 3080 hashrates in the 60s or low 70s and 3090 in the 90s. They are neither low tdp nor cheap, so to me this doesn't represent a value proposition. But ymmv.
It will be better to buy two GTX 1660 super that will give you 60MH/S than buying RTX3080 than will only produce 60+ MH/S for insane price, I'm guessing the price will start around 2800$ or more, this can get me 7 to 8 GTX 1660 super 😎