I'd invest top tier BotW money and here's why: the demand for the game is huge, that's obvious by the reactions across the gaming community when it was announced last E3. Moreover, it has the potential for hype to go through the roof if Nintendo treats it like their highest tier games and really knocks it out of the park. It has a hugely popular platform to launch on with an install base that will likely be approaching 50m when it releases (assuming it's at least a year away). None of the Prime games have ever really had this since the GCN install base was never very big, and Wii was still in its infancy and only 9 months old when MP3 came out. Now I know the counter argument is going to be that Metroid has never been a huge seller so why would this?
The answer lies in games like Skyrim, BotW, and most recently God of War. All of those games absolutely destroyed the previous sales ceilings in their franchises because the respective studios went all out for them and delivered products that blew people away. This made the hype and word of mouth explode for them.
The same thing can happen for MP4 if Nintendo plays this right. Again, the insane reactions over the reveal prove the potential for massive hype is there for the game. But they have to be willing to treat it like it's a Super Mario game or something, much like Sony treated the new God of War as if it were a GTA level of popularity game. And it paid off huge, the game could come close to tripling the lifetime sales of the previous best seller in the franchise. I can see a similar situation for MP4 where it does around 3x MP1's franchise high for 7-9m lt if they invest in and treat it as a top tier exclusive. Also, they should steal a page from God of War's playbook and drop the 4, that could hinder its ability to appeal to as wide of a new audience. Just give it a subtitle.