I'd rather get charged for the license as part of the cost of Sonar (and have that go up a few bucks) then get asked to upgrade every time I forget that Sonar doesn't offer this pretty basic feature stock.
If you're willing to pay extra for it, pay the twenty bucks and then you'll have it forever.

Better yet, use the $20 Sonar just gave all registered users and use it to pay for the MP3 encoder.