DO has bandwidth included in the price of the droplet and if you exceed then you pay a fair price for the additional bandwidth. For the cheapest droplet ($5/mo) you are provided 1TB of bandwidth. What they also do is take the all the bandwidth across all droplets you purchase and make that your pool. So if you have 4 droplets, then 4 TB, if one of the droplets experience the most traffic like 3.5 TB, and the rest are zero, you are under your total bandwidth quota for the billing period.