Those buttons look like pop'n music buttons, but only two on a controller instead of nine. Considering an arcade sized pop'n music controller starts at around 200 USD, a Divekick controller should be possible for $50 or so, certainly less than $100. Perhaps they should kickstarter the controllers.
$50 as the baseline is too high. $20-40.
I'm pretty sure the baseline costs of manufacturing, shipping, and retail all add enough middlemen that $50 is probably the lowest amount of money you could hope for. Maybe $40 if they really chintz out on the materials.
If it comes with the game (hopefully on Steam, come on Valve peeps, get on it!) I'd pay $100 for two of them. If they do a Kickstarter for them maybe they could do them for less? Since they'd know exactly how many they'd be making? I'm not sure if that's feasible, but it sounds right.
I am seriously contemplating getting one of those stupid anime boob mousepads and turning it into a DiveKick controller.
My biggest obstacle is the shame of having an anime boob mousepad on my Amazon purchase history, and the implications it will have for my "Maybe you'd also like to purchase..." suggestions.
*Edit: As for the actual costs, @warpr, I've been looking into that stuff and those 100mm dome buttons are generally around $8 each. So with 2 of those buttons, the materials for the box, the input controller, the overhead for actually building the things, and some amount of markup so that they can make at least a little profit on each unit, my uneducated guess is that somewhere between $40-60 would be about right.