Considering even Apple doesn't get any meaningful profit from its app store, I don't see Android Market being a profit center either. They need Android Market just to be competitive with the App Store.
Apple's app store revenue is around $2 billion. If Android market can generate a similar amount of revenue for Google, it should a good share of the total Android revenue.
What good is revenue with no profit? Apple has said that app store basically break even in terms of profit, so I'd venture to guess Android is similar. Although they don't need an army of approvers so maybe their costs are less. But they most likely don't make as much from paid apps or in app purchases as the app store either.