Darcy knows surely - if he is any judge of character at all -that Jane is not the type to marry a man purely for mercenary reasons.
If she did want to marry Bingley,without being in love with him, she would certianly have tried harder to attract him, to show, discreetly but definitely, that she was interested.. YET she was so calm and proper in her demeanor that it was possible for D to convince himself that seh had no feelings for Bingley. It seems to me that in his attitude to Jane, J is damned if she does and damned if she doesn't. If she had shown feelings, he would have been disgusted that she was being "Unmaidenly" or suspsected her of exaggerating to win Bingley's affection. Since she didn't show anything, he just decides that she does not love B at all, although she is simply acting a "proper" manner. AND even if she did not love Bingley but might have accepted his offer, it was not D's business.