But, how much bandwidth does the daemon use a month?
Very little at the current level of activity. The growth rate of the blockchain is currently something like 2 MB per day, so still well under a GB per month of bandwidth on average, assuming each transaction is sent four times (once in a block and once on its own, sending and receiving).
Even low-end mobile plans can cover that within base usage. (I don't recommend that though, since there may be substantial variations from the average.)
So a growth of 2 mb/day is equivalent to 1579520 b/day in natural growth, or 1.580 mb/day in legit transactions, where we have about 1.580/.010 kb /tc so we have like 158 transactions per day, or the equivalent of .001828 transactions per second, or about .1828% of bitcoin daily transaction action daily?
We're obviously dealing in gross generalities here, but it seems close. Someone claimed btc at 1 tx/sec. That would be 60 tx per 60 minutes Monero block. Look at a Monero chain explorer and you see well under 1 tx/block, maybe 1/10 tx/block. So that's close.