Pick the MAX you would be ok with. Ok as in feel like it is a fair price.
Mario Kart 8 is pretty awesome, but the battle mode that is just battling on the regular courses... sorry, weaksauce. I don't particularly want to pay more money for a battle mode that should have been there but... I will! ADD A DLC BATTLE MODE NINTENDO!
By "traditional" I mean with the type of battle-specific maps that we have seen in the old games. And for the sake of argument, let's assume that there would be about 5-8 maps or whatever also like the (recent) old games. Including a few retro maps (Block Fort!) And maybe 2-3 varities... balloon battle as one, maybe bring back bob-omb battle and some new thing, I dunno.
I've gone on record many times saying I'd pay $50 for just Mario Kart 64 battle mode with online added. So I'd be happy to pay that if it reached those heights. Unfortunately, I don't trust them to since there hasn't been a battle mode anywhere near as good since.
I'd pay the equivalent of five bucks for four arena style battle mode levels, preferably including a version of Block Fort. I suppose I could have voted for prices upwards of ten bucks if they cram even more levels in there, but I think a first pack with four levels at about a fiver would be a good place to start.
I would probably pay $40 for what is essentially Mario Kart 64's Battle Mode in exact perfection plus HD. Because I loved it that much back in the day.
I however have complete doubt they could make it that level of brilliant at this point. Not after so many failures. I don't even think they could copy and paste the original code successfully, they'd mess it up somehow.
I'm really good at wasting money, so spending twenty or so on something that would be fun works for me. I'd also pay money to have an options menu in MK8, for being able to adjust the volume on music/SFX/etc, and other such settings.
@Mr_Mustache It'd be tough to say whether this theoretical battle mode would be worth it since it would depend on how Nintendo managed it, but Mario Kart 8 is definitely one of my favorite Mario Kart games in track design / racing / etc. so if they could add that attention to a battle mode it could be great. Of course, some hate the new item balance so YMMV. And only holding one item at a time might make battle mode lacking a bit... I like it for racing and I know it used to be that way in battle mode long ago but I think getting to line up multiple items is better for battle mode.