It is absolutely possible to have the phone connected to the charger as you use it as a hotspot. If you don't have a big enough data allotment I guess it's not a great option though. Thethered computers use a lot more data than when you use the phone itself for surfing or whatever.
Over here we have PayByTheDay prepaid data cards. Maybe ther is something similar available to you as well? Like what LoveMacs suggest.
Edit: Ooops! I see now you posted while I was writing!
I'm so slooow...