i could be wrong here, but it really looks like ur biggest bandwidth usage would literally be ur message size on received mail....the keep alives and synching would be close to minimal?
i have not used MS direct push, but i sync my other mobile accounts (mobile me etc) as push mail 24/7, the usage is tiny.... I dont see why it would be any different....but maybe someone who uses it could confirm..
Reckon easiest way is to literally connect and monitor usage