I'm having a bit of trouble gauging the power level of this card, but I am inclined to say that it is too powerful. I suspect that most decks running many artifacts are well prepared to pay for them already. However, whether they can pay full price or not, you're forcing a discount onto them and them and them setting up a system whereby if the number of artifacts they control ever exceeds their mana generation, you get to start stealing artifacts. In the meanwhile, you're keeping their mana tapped out and if/when they
do give up an artifact to you,
you get to choose what to take.
One way in which this card might be balanced a little bit might be to say something like, "When an opponent plays an artifact spell, he or she may pay
less than that spell's mana cost. If he or she does, that artifact comes into play with a debt counter on it." Then, keep the rest of your card's ability the same.
That way, if they
can pay full price and wish to do so, they don't go into debt to you. I am also pretty sure that the above wording dodges the problem of putting debt counters on things discounted by other means (Helm of Awakening, for example). It also doesn't put a debt counter on artifacts brought into play by other means, such as Tinker or Tooth and Nail.
Finally, the flavour of this card simply isn't red. Red doesn't steal artifacts, typically -- especially not permanently. Steal Artifact was reprinted in 8th Edition, so there's clear evidence that permanent theft of artifacts is still a solidly blue effect. Finally, the idea of loan-sharking and racketeering seems much more blue or black than red. I would be comfortable seeing this card being a 1/1 Wizard for
or
{U}. It's a shame that Cephalids were a one-time creature type. Loan-sharking seems exactly the sort of thing they'd be doing.
