Did they really say they would replace the game for $80, or did they say they would replace the game and the Wii for $80? I somehow doubt they told you they would replace a $50 game for $80. I'm not saying you're unreliable, I'm just saying that Nintendo would be pretty freaking stupid if they actually did that; so freaking stupid, in fact, that I am inclined to believe you are mistaken. You are, after all, just as fallible as the rest of humanity. If they really did say that they'll replace your $50 game and nothing else for $80, you should really call them up again, because someone had to have made a mistake somewhere. They're fallible too.
But if that price is for both the game and the Wii, I'd recommend doing it, even if you don't think, in your sole estimation, that the Wii is the problem. From where I'm standing, there's every reason why it very well could be. Brawl is probably the only dual-layered Wii game you have (You may have another one, but I had no way of knowing that, so if you're going to inform me of that, please be polite about it. I'm trying to be nice here, but if you don't want my help, just say so. I'm certainly not getting paid for this.), and, as I said before, there are many, many reports of Wii disc drives that work fine with single-layered discs but are damaged just enough by various airborne particles that they can't read dual-layered discs (Since you didn't claim to know that wasn't the case the last time I brought it up, I'm assuming it's still a possibility, at least until you address it somehow.). Nintendo gives you a new 1-year warranty every time they repair something, so it could end up paying for itself (And really, it's a pretty good price anyway. The standard used to be about half the price of the console for unwarrantied repairs.). If the Wii really isn't the problem, then either you just got a warranty for $80, or they probably won't charge you for it, as far as I know. But if you just go out and rebuy the game, and the Wii really is the problem, you'll be out $130.
If, after intently reading that wall of text, you're still certain that the problem is just the disc, then screw Nintendo and go ahead and rebuy it already if you care that much. But don't say I didn't warn you. This is where, if I cared, I would ask you how you're so certain that the Wii isn't the problem, and then we might be able to continue, but I'm exhausted. I've said all I'm going to say.
Goodbye.