Garmin today launched a one-time purchase plan for its Garmin Mobile for BlackBerry service, offering North American BlackBerry owners unlimited its GPS service for $99.99.

Sounds cool, until you find out that the deal is for the "life" of a specific BlackBerry device, and can't be transferred from to a new device. Considering that most mobile devices usually have a two-to-three year lifespan, the $99.99 plan translates to a service plan of about $2.78 to $4.17 per month, again, depending on how long you can keep your BlackBerry alive. Keep it longer, and the price-per-month drops even further.

The service offers turn-by-turn directions, as well as unlimited access to Garmin Mobile's dynamic content, including traffic, weather conditions, forecasts and fuel prices.

If you own a BlackBerry that doesn't have embedded GPS, you can buy the separate GPS 10x, a portable GPS sensor (or "puck") that receives the GPS location data and sends it to the BlackBerry via Bluetooth. The 10x is not included in the one-time purchase, Garmin says.