I wouldn't say it's ALL about the money, but that's a big motivating factor, yes. Mathematically missing out on the Champion's League places just gave Man Utd the excuse to fire Moyes. By all accounts the actual decision was made months ago when he reportedly lost the dressing room. Once the players don't buy into the boss, it's over.
Here's a guy that is near-universally respected by his former players at Everton, who was apparently a tough taskmaster and could really deliver a hairdryer at halftime, who got his teams to play with attack and verve....yet when he gets to Man Utd, he's run out of town by players who immediately hated him and saw right through him. I guess because he was the only guy in the room who hadn't won a trophy.
I feel for him, but he never got it right. Clubs like Man Utd need a winning ego as a manager above all.