Let me turn the comments into an answer:
The "irony" has more to do with the way Hadley leaves the money rather than with the $10 bill itself.
You see, while Hadley was fascinated with Martha, he is shocked when he finds that she has been sleeping with the charismatic black leader of the religious movement. In his twisted mind, that means that she is (forgive my language here) a "whore". Hence he treats her in a way that he believes that whores should be treated: he has sex with her, he beats her up and... leaves the money (for the taxi) on a dresser.
Leaving money on the bedside table/dresser etc after having sex is a typical way of treating sex workers. Leaving only $10 (enough for a ride on a taxi) suggests not only that Marsha is a prostitute, but also that she is a cheap one.