Color me skeptical. This is a Punisher card. Your opponent will choose whatever is best for them. If you're locked down under a Sphere, they'll rely on Sol Lands instead.
If it cost 1 mana, then they probably would not have a choice; they kept a hand with a mox or crypt and need to use it, probably. So, if it cost 1, it would situationally be as good as Llanowar Elves. That's not high praise.
There's a second use case, along side Welder or something else to sac the artifacts, but that's an awful lot of work and mana to do what Mox Monkey already does.
It's important to note that, thanks to Paradoxical Outcome being an instant, they can respond to all of the triggers that Treasure Nabber creates much in the same way they respond to tapping their own Sensei's Divining Top.
It seems naturally costed for a Moon Stompy shell. That seems like a fun avenue to explore that might have a natural payoff for sudden bursts of mana.
This card is either very good vs paradoxical outcome, or it is terrible depending on rules interactions.
If a player announced paradoxical outcome, pays for the cost of it with 4 Mox mana, how many moxen can they target with Outcome before the triggered ability from this card resolves.
I’d assume since the spell was announced, that all the triggered abilities would go on the stack simultaneously, and they could cast outcome targeting the 4 moxes before the triggered ability resolves.
But I am not sure.
@gkraigher if you just announce outcome and tap your moxen to pay for it, the triggers resolve first, stealing the moxen, and they don't bounce and you don't draw.
the proper way to do it, from the outcome side, is to activate the mana abilities(and anything else, like top, that must precede outcome) and then cast outcome in response to the Nabber triggers. all bounce, all draw, none borrowed.
It's not really that big a risk for the outcome player. If you play outcome and your opponent counters it with this card in play, they get your artifact mana for one turn, and then you get it back next turn and try again. That's exactly what you were going to do if they had a counter and you DIDNT have a Treasure Nabber - it doesn't even really slow the outcome player down
It's only scary if your opponent has something to do with that extra mana, in which case a Mana Drain seems far more useful for them.