Nice. A sword that comes with a blacksmith attached. There's a surprising lack of abusable targets for this. Elbrus, the Binding Blade is one. I guess there's a fair few where it gets around colour costs. (Unscythe, Killer of Kings....)

But.. what does this do with cards with multiple equip costs? Does the card have to meet the equip requirements? Something like Steelclaw Lance?

I think I like this.

Modern day design seems to be moving the cost towards equipping rather than casting, which makes this harder to abuse. It's easier to cheat artifacts into play than attach them for free. There are certainly some older cards that you can cheese with this.

When a card has a specific equip ability like Steelclaw Lance, I think this gets around the targeting restriction, so you could pay and attach it to any creature, not just a knight.

I looked at equip costs 0-2 and literally only found that one example where the casting cost was much higher than the equip cost. If you're willing to step up to higher costs there's more, but even then the margin of gain is fairly small. I was kind of expecting to find at least a few "Huge up front cost, teeny tiny cost to use it" but there really aint that many.