So many great thoughts around this ... the only thing I'd add is that a decision around solely financial principles will fail in the long term. Systems are more important than goals and Core Values alignment are more important in the long term than short or medium term promised or proposed financial gains.

Looking for Core Value alignment first empowers the freelancer to truly create synergy and leverage in the new relationship. Gaining 20% in something that has synergistic value in addition to a reasonable base may make sense if the partners are in great alignment. If not, there's no long term value to the relationship.

Tactically, the freelancer should look at the agency's systems and processes to ensure they are efficient and that there's further operational value in the alignment of their priorities.

I've written a great deal on core value alignment and you can find blogs and podcasts here: http://wisenapkin.com/tag/core-values/

Good luck and let me know if I can help.