The difference between $300 and $800 is the difference between me buying one as a development tool and me not even bothering to do "Metro" development. At $300 they'll be a viable competitor to the iPad. At $400 they'll probably do all right. At $800 they'll sink like the Titanic.
Microsoft's little experiment can succeed, but not with the device prices we're already seeing from OEMs. Asus are out of their g-dd-mn-d minds. Lenovo's ThinkPad is a better deal if the keyboard it ships with is the same as their current ThinkPad laptops. The rumors about Acer's pricing are all over the place, but I'm not optimistic (which is sad, 'cause I think the W510 is adorably cheesy).
My favorite part of all this is is how it seems like Windows 8 tablets are topping out at 4GB RAM when the WP8 emulator recommends at least 8GB, and the absolute cheapest device with a non-Atom Intel processor (that I've seen) is the Acer W700 at $800 (which doesn't come with a laptop dock, which kills it for me). It's kind of bull$#!+ when you consider that people will probably want to develop for both phones and tablets at the same time...