Lex making the decision in favor of money and power to secure his happily ever after is like the saddest and also truest moment we get to see with him.
He knows that the friendship with Clark was the closest thing to the genuine love he's longing for that he ever got. (which makes it even sadder that he said to Lillian's ghost that the only people he ever really loved were Julian, his mother and Lana - I don't doubt it that he also loved Clark but it's so telling that this one friend isn't mentioned.) But I am not sure that Lex knows that Clark wasn't his friend because of money and power and sports cars but because he really liked the sarcastic, intelligent and sincere Lex in search for his way of being good.
Because we only ever got to see him use money and power to help Clark and his family and therefore show them what they mean to him. So this is the only way he knows to express love and to secure this love because well, Clark kept loving him and Clark is still coming to him for help even though they are more or less done with each other. Lex never experienced that people could love him unconditionally for what he is (and Clark couldn't express that he did) - a love which leads to a feeling of safety and stability. A feeling Lex only ever got through the use of power and money.
So the Lexmas dream gave him all he ever wanted - safety, stability, unconditional love, happiness, living somewhat carefree. And he knows that money can buy him safety and stability.
Lex choosing money and power in his need for love and stability is his true self, which makes it really sad because the poor little kid thinks that he will be loved because of this.
And to make it truly heartbreaking: Lionel fucked it up by taking the two people who would have loved Lex unconditionally - Julian and Lillian - away from him. And Clark failed spectaculary at expressing that he isn't friends with Lex because of his wealth (and I think Martha liked Lex too and couldn't express it the way Lex needed it due to Jonathan). And I believe if only the Kents were able to show Lex what unconditional love means, things would have taken another turn. Him hesitating to choose this way at the beginning of the episode showed that he knows it in his brain that money can't buy love. But he doesn't feel it in his heart and after all, his whole arch so far was about his search for his heart (as in the right way for him) and love.
But if Clark told Lex his secret from the start and that he loved him, we wouldn't end up with a fucked up evil Lex and then what would be the point of the show if not torturing Lex and fucking him up? Like it could have been really intriguing to see Lex grow and we end up with the powercouple Lex Luthor and Clark Kent/Superman. Could have made an actually good show.
