Just for the fun of it my data - from Estonia:
- i pay ~$5 for "unlimited" data (Actually my speed will be capped to EDGE speeds after 5GB - but i never use that much)
- i have chosen to pay by the minute (~$0.02/min). That rarely exceeds another $5 as most my calls are free (family+coworkers)
So all in all about 10$ a month.
Also - i practically never use SMS/MMS, but jabber/facetime/fb chat/hangouts instead.