I can think if a couple of examples of cards that this breaks (or at least damages.) At the very least, we'd need to know when during resolution you have to pay costs — that is to say, before or after the card's effect takes place. If after, then consider the following examples:
Dark Ritual, Seething Song, etc. — You can play it with no mana in your pool and use some of the mana generated to pay the card's cost.
I indeed wanted this to happen before the spells effect.
I'm curious, though. How exactly do you see this making casual more interesting? The point seems so subtle that I can hardly see it mattering in many casual metagames.
Actually, in a ton of casual metagames, there's always that guy who plays 1290381 counterspells, with the most retarded win condition (like 1 stalkingstones or something). This card would pretty much wreck that guy, since you could play your threat, and if he counters, so what? you didn't pay yet. It's basically an artifact that makes instants worse, rather than one of the infinate cards that makes instants better.
As for templating, I'm hoping some rules guru can help with that.