I don't believe windows 8 phone will improve the market share, there are 3 potential mobile marketshares are: Android, iOS and Blackberry.
Obviously, Blackberry 10 will come out in 2013 which could beat Windows 8 phone
IMHO Windows phone 8 will sell a lot more than BB10, but it's only my sensation. Anyway, $8 for a developer account... it's really a good deal comparing to apple policies.
Now if you could only find us a deal like this on a new Windows 8 phone, and a desktop and tablet windows 8 developers license <img src="smileys/smiley36.gif" border="0" align="middle" />
There's a low cost HTC model (8S) with windows phone 8 planned. This may be a good platform test device.