Using the computers I already have, I get ~$20 a month in Gridcoin. It's small but somehow satisfying, since all it costs is electricity. There is no baseline investment in hardware.

What surprised me at first is that mining with my CPUs was actually MORE efficient that GPU mining. This is because some of the scientific projects on Gridcoin are CPU only, which means you are only competing against other CPUs. If you have a Desktop with a nice i7 processor, that alone will take you a long way.

Not only that, but mining Gridcoin doubles as philanthropy (donation of processing power to science).

At least, this is my experience. Happy to hear about other people's experiments with mining altcoins.