I still have an old Sprint Data Plan (Business) with no data cab or extra charges and I burn on average about 30-35 Gigabyte per month. Emails and a lot of work related documents make eat up my data.
My Sprint TP2's (two units) are used to stream Music or Slingbox in the car and for tethering if needed and eat up about 20-25 GB.
Total last month: 82.5 GB and no questions or complains from Sprint. I love them for it and I know why my business stays loyal. I guess they hope to move me to 4G soon in LA
My TMo TP2 eats about 10 GB and last time I talked to CC, the guy was quite surprised about my data usage.