A hetero-GPU setup is definitely cost-effective if you don't strictly require the raw speed of a top-tier card like 5090. Just keep in mind that the total throughput will also be bottlenecked by the slower card.

To provide some anecdotal data, here is how my 5090 + 3060 setup performs with Qwen 3.8 27B (Unsloth's UD-Q4 with MTP):

  Single 5090: 101 t/s (TG), 2650 t/s (PP)
  5090 + 3060: 53 t/s (TG), 1700 t/s (PP)

For reference, here are also some numbers from my 4060ti + 3060 (16GB + 12GB) setup. [0]

[0]: https://news.ycombinator.com/item?id=48700091