Garmin provides free map updates within 60 days of purchase for new units if newer maps are available than were included with the unit. After I registered one with 2008 maps on disk, I was able to order free 2009 map update DVD and got free map unlock code online. When I bought another model with 2008 maps, all I needed was a free 2009 unlock code online, since I already had the update DVD. Software updates for the unit itself are always free.
If you get a Garmin nuvi that already has current maps preinstalled, you may need to pay $15 to get maps on DVD (for your PC). But if maps are outdated, they will give you free update (download or DVD).
I have no experience with TomTom, but would expect something similar.