Why would a phone 6-7x the price be roughly equivalent in experience? Why should it be? The 640 isn't subsidized by anything. $99 is the just the price of it. It's an amazing device, FAR better than an equivalent android.
People complain about Windows Phone not having "flagship" devices, but realistically, it doesn't need them. The cheap phones are great.
I've tried expensive android phones, iphone, as well. None of them stand up to the experience on WP for me. If Microsoft keeps making $100 phones like this, I'll never pay more than that for a phone.