The license cost starts at 20k/year with 1.4 USD license fee per 1000 views (CPM) for websites and mobile apps. The licensing of 1.4 USD CPM alone is more than normal sites can hope to earn from 1000 page impressions.
If I were RapGenius I would welcome the payment because it would clear out a ton of competitors and duplicate content in one fell swoop.
The license is probably not entirely static, most likely based on traffic like paid web fonts. My guess is that it would be significantly more expensive for Rap Genius.