It's his Strikeforce contract. SF paid accordingly.
Accordingly enough to not sustain themselves.
A lot of people keep saying this, but Robbie's payouts were a lot different. I would guess only the Koscheck fight was under the previous contract. UFC doesn't typically do big guarantees outside of guys with PPV money:
From AC payouts only:
Strikeforce
vs. Mahoef $100,000 (No win bonus)
vs. Sobral $85,000
vs. Souza $65,000
vs. Amagov $150,000 (No win bonus)
UFC
vs. Koscheck $95,000 ($10,000 win bonus) ($50,000 KO bonus)
vs. Voelker $78,000 ($78,000 win bonus)
vs. MacDonald $83,000 ($83,000 win bonus)
vs. Ellenberger $100,000 ($100,000 win bonus) ($50,000 POTN bonus)
Seems pretty obvious to me he got a pretty standard zuffa "fight for half your paycheck" contract in his second fight. Surprised so many are sayng he would still be on a Strikeforce contract. He is five fights removed from the promotion at this point.