As for prices, I was referring to the MSRPs, where the MB-D10 is $330 versus $307 for the -D15. It's nice to know the street price for the -D10 is more reasonable.
ttc546, vitalishe: As far as the price, I'm pretty sure economy of scale has a large influence. Suppose that for every 100 D7100 buyers, there's 1 who also buys a vertical grip. That means that Nikon's MB-D15 line and supply chain will be running less efficiently than the analogous line and supply chain for the camera itself (imagine if they can fill a shipping container with cameras, but need to share containers for the grips, as one example — they incur extra handling costs just to combine shipments).
This isn't to say that the MB-D15 is more expensive than it should be; but rather that the D7100 is likely _less expensive_ than it would be if produced at the same volume as the MB-D15.
That said, is Nikon charging a hefty markup? I'm pretty sure the answer is "yes." Is that reasonable? I'm inclined to say "yes" also. Keep in mind that they need to recoup their design and development costs for the grip. (At the same time, this would be a good reason for them to figure out some way to share grips between bodies, although it's unclear what body it could share with in this case — there's nothing out there with a similar body size and shape. The D7100 is physically larger than the D7000, so the MB-D11 is right out.
That's also why I don't think it's fair to say "oh, a grip should cost $x00." Just like the value of a photographic print is more than the value of the paper, the value of a grip should be more than just the cost of its constituent parts. R&D, design, QA — it takes money to make a consistently good product. I think it'd be telling to see what the industry would come up with here if Nikon hadn't done most of the design and engineering for them.
Either way, to each their own.
------------------------------------------------------------ I make prints of moments that people love.