Salary
Average Salary: $25,000
Expected Lifetime Earnings: $1,043,700
Bike messengers typically get paid by the delivery, which averages around $4 to $5 per piece (source). If you complete twenty-five deliveries at $5, that gives you $125 a day. It's not awful money, but you probably won't be retiring on it anytime soon.
Find yourself with a slow day or a less-than-popular company where you only get five or six calls a day, and it's Ramen noodles for the week.
Some companies pay hourly, but rarely above minimum wage.
Other companies offer cash incentives for the fastest delivery times, which ups the danger and the pay for certain jobs (though messengers who take these are usually the most experienced and enjoy the risk).
The average take-home pay per week is between $300 and $400. A tiny handful of elite and highly skilled messengers can make as much as $600 to $700, though this is rare (source).
You'll also be required to own your bike and keep it in good condition. Actually, you should probably have more than one bike, and you'll also need to replace tires often. They aren't cheap. You'll need a helmet (always) and a good bike lock, because while you're delivering that important package, someone could be getting a five-finger discount on your ride.