The "additional fees" I assume you're talking about are the BB server fees. The BB server is needed for the BB's propietary protocols, ie messaging and internal browser, but not for anything else (which is why opera mini works). Tethered data does not use the servers, and therefore makes no difference.
You can tether the average blackberry out of the box, but you'll have to pay extra for a PAM plan (Phone As Modem) with Sprint, otherwise you'll have to pay per kb of data. Contrary to popular belief, PAM is not included in unlimited data with Sprint.
There are ways, however, to prevent Sprint from being able to tell the difference between phone data and modem data.
In order to tether undetected, you need to disable something called multi-NAI. On samsung phones, this is in a hidden menu, on WM phones its an app you can run, however on some phones you are simply locked out and can't adjust the NAI settings.
I'm not sure how NAI works on Cdma BB's, but that's the info you're looking for. It may be possible, or you may have to add PAM to your Sprint account.
Good luck!
Edit: Just wanted to point out that if tethering is important to you, the BEST data devices to tether with is a WM or Symbian device that has Wifi. Both OS's have an app available that will route your cellular data connection to a peer-to-peer wifi network, effectively turning your phone into a portable wireless router. You can then connect your laptop, psp, ninetendo wii, etc, even at the same time if you don't mind a little lag (and possibly a prematurely dead battery!).