I don't recall the exact amount but I think Garmin charges around 70 bucks if you just purchase a single update. For a few dollars more you can purchase a lifetime access that will let you do up to four updates per year. At least that applies to my Streetpilot C340, so I assume Garmin offers the same update options for all models.
I've encountered very few places where the Garmin was out of date. The glitches I've found are usually around some cities where orange barrels are a year round decoration and bypasses have been added or rerouted and/or exits have been changed. Cross country routes don't seem to change that much and the GPS is usually correct. That said, mine has given me some strange routing on a couple of occasions. It's always correct and gets me to the right place, but not always by the best route.