Go to http://www8.garmin.com/updatemaps/ and look at their official page. Typically with a newer unit (under a year old) you get a single free map update with your Garmin GPS device. After a year you are pretty much stuck with purchasing a single upgrade or opting in to their forever-update package for about $90 bucks.
Also take in consideration how old your Garmin GPS device is. If it's a few YEARS old it may simply be more cost effective to purchase a new Garmin GPS outright that comes with a new map and a free update rather than just purchase the map update.
Go to http://www8.garmin.com/updatemaps/ and look at their official page. Typically with a newer unit (under a year old) you get a single free map update with your Garmin GPS device. After a year you are pretty much stuck with purchasing a single upgrade or opting in to their forever-update package for about $90 bucks.
Also take in consideration how old your Garmin GPS device is. If it's a few YEARS old it may simply be more cost effective to purchase a new Garmin GPS outright that comes with a new map and a free update rather than just purchase the map update.