Do you have ideas for mining equipment in the $600 area for ETH mining beside building a mining rig?
You can still hobby mine with a mid level CPU based gaming rig with a decent AMD card IF you have very cheap electricity. It may not earn much, but should be able to pay for the the next upgrade of your current card. You can't really do this with lend end cards as they don't have the memory speed/bandwidth to support ETH which now needs 4GB+. Something like a 5700 XT with $200 for the rest of the rig should work. You can get in some gaming in down time.