Mmm, a drawback Sol Ring. Still too good to be printed these days, I fear.

make it cost or maybe

At it'd look a bit sad next to Palladium Myr (although that has summoning sickness). would probably be the right place: clearly powerful, but not ludicrously overpowered. Like a Joraga Treespeaker, if you pay 2 mana on turn 2 it'll give it straight back; the damage makes up (ish) for it being nongreen.

I was actually thinking about Ancient Tomb, one of my favorite cards.

Ancient Tomb is much better than a typical land, so why would you expect its fair cost in artifact form to be less than ?

Mmm, good point. It's funny because I play a lot of black and my friends play green and white and say "This card is really lame. . ." whereas I say 'turn 1 swamp, turn 2 ancient tomb, Phyrexian Negator, done.

I think this is not common.

That's a good point: nothing that makes repeatedly taps for multiple mana has been common for many years.