"is it possible to get a gps in a prius second generation"
Maybe, I am not sure if a global positioning satellite would fit in the car

However the 2G (NHW20) Prius does have an optional DVD based GPS navigation system. The map DVD drive is under the driver's seat on cars so equipped."And if you have a third generation with gps inside how much do you pay a month? approximately"
There is no recurring charge for the navigation system beyond possibly coughing up a few hundred dollars every few years to buy an updated map DVD, however this is optional. How much the option affects your car payment is another story.