In John Gruber’s initial thoughts on the HomeView, he wonders about the odd $300 pricing for both the regular HomeView and the wall-mounted version:

So the wall-mounted version, when you include a mount, costs more but sounds worse? This doesn’t make sense.

I was thinking about this too. All I can come up with is that Apple doesn’t want you to buy the wall-mounted version unless you’re really sure that’s what you need. If the wall-mounted version was cheaper, some people might buy it based on price, then be unhappy because it’s too much trouble to install.