I'm going with free. Nintendo has already said that their online services are going to be free (as the DS WFC is now), you won't have to pay for the internet access because you will already be paying it to your ISP
Opera for DS Lite is $80USD it might be cheaper. The only reason we have to pay for the DS Lite version is that it isn't already preloaded into the system like Wii. My signature sends u to the Opera for Nintendo site.