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.