You're off by a factor of 8, bandwidth is typically measured in bits per second while data is typically measured in bytes. Also, saying that they only have DATA_CAP / BANDWIDTH amount of time to use the internet is disingenuous at best, the only time that you're using 100% of the available bandwidth for any sustained amount of time is when downloading a large file. 21 GBs isn't a ton of data but that can easily last someone a month if they're just using it for web browsing for business purposes only. Online backup is out of the question but for just email and web browsing that data cap isn't too big of a deal.
Honestly, if your options are share a 3Mbit connection with a few people or have a fast connection limited to 21Gbytes / month I could see paying for that out of pocket.