This ability is descended from a keyword for artifacts until I found that to be too problematic for such a status so I had to reduce it to a single card.

Abilities don't have "mana costs", just costs--and by phrasing it in a manner that actually works, the effect quickly expands from targeted to global if you make sure to target itself with the first activation.

I think it's pretty clear in intent - how about:
UETO you may pay instead of the mana needed to activate target artifacts abilities. (Non mana costs must still be paid)

Saying that - it's still utterly utterly crazy. Infinite mana with Basalt Monolith etc. infinite damage with Rocket Launcher and company... hmm; doesn't sem to horribly break life or card gain; those are mostly one-off or .

Alternative: "Add " to your mana pool, use this mana only to activate the abilities of artifacts. That makes it REALLY obvious what you're supposed to combo this with :)

­Staff of Domination is lifegain and card draw in one, and it really likes having this around. Then once you draw your damage artifact, you can use Staff to untap this, and activate it again targeting the new artifact.

I would be tempted for this to read ": Activate all activated abilities of another target artifact without paying their mana costs."

Oho. Link's suggestion is a good one.

Actually it should read "Activate all activated abilities of another target artifact without paying their costs," as SadisticMystic pointed out.

Staff of Domination is still a runaway winner that way, besides the obvious Door to Nothingness, Time Vault which is already too easy, or Kuldotha Forgemaster to fetch any of those game breakers.

More complicated routes include a second copy of this and something that gives one of them another activated ability like Hermetic Study, or Filigree Sages + Illusionist's Bracers + any other artifact with a useful tap ability.