Blackberry is not going to give you anywhere close to as cheap of a data plan as a WM phone. Blackberry is pretty expensive, and will cost you much more than a WM tether option. However, I don't think tethering is even possible on the Q9c without 3rd party software, which I'm not sure you could even install.
You're not going to find an incredibly cheap option that allows you to tether the connection easily. Sadly you usually have to pay well for that option. Either by data plan or otherwise.
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