$100 per submission is a pathetic payout for any given app of a decent quality. Obviously, the $100 is not supposed to fund the development of your app by itself. It is only meant as an incentive.
Still, the upfront feebleness of the reward doesn’t help Windows Phone’s PR at all. It feels like a move a company makes in desperation, when they are clamouring for marketshare but do not have any money left for marketing to reverse the decline. Sadly, if it wasn’t for cash-cow Microsoft financing the unit, that hypothetical situation would apply to Windows Phone now.
A much better strategy for Microsoft to boost Windows Phone development would be to seed key indie developers with hardware. Sponsoring free sales of apps is another alternative use. In fact, take the million dollar kitty and split it equally between the top ten developers on iOS in exchange for their commitment to the Windows Phone platform.
In essence, instead of giving a negligible amount to many, do stuff that gives a reasonable payback to some.