I don't think Tmobile charges extra for tethering, I could be wrong though. If I'm right, then yes, you wasted $50...
The only way to get 3G speeds is by using a phone that supports 3G. Your 8900 will only do EDGE. Tethering is defined by using your phones internet connection to browse on your computer. So whatever speed your phone works at is what speed your computer will work at.
So what's the issue then?
Your phone isn't connecting to 3G when it's connected to your computer?
Have you tried tethering in different places? Like maybe the service is bad where you are.
Does your phone normally stay in 3G mode even when you're not tethering?