Its $39.99/mo for unlimited usage (requires 2 year contract, and with a voice plan). Adding unlimited text is an extra $5 with the bb plan. Another plus (if you have an evdo device) is with the bb plan (plus a voice plan) you can tether your blackberry at no additional charge. Good deal IMO.
Of course I'm talking about the market I'm in (los angeles) things might be different in your market.