I think Jenny betrays Mac for the money, plain and simple. There is the usual comparison between she and Judas and the analogy/theme of betrayals. Being one of his former lovers, she might also be seeking revenge or she might be jealous - she does still reveal where he is even after she doesn't get paid - though I don't remember this explicitly being in the text. Of course, it doesn't have to be there in an obvious way; it is up to interpretation of the reader.
Overall, I thought the play was about money, selling out, being bought off: beggars, prostitutes and thieves - and even officials can be bought off. So, in addition to other possible motivations, I'd say the main motivation is greed and self-preservation. To an extent, I sympathize with Jenny. In the Marxist interpretation, she is caught between the dialectic between the Peachums (bourgeois) and their employees (proletariat). I see the analogy between her and Judas, but there the connection ends. Like most of the characters, Jenny is part of a cyclical system of betrayal. If I had to pick ONE motivation, when she says Mac is with Suky Tawdry, I guess it would be a mistake made in frustration. When Peachum denies her the money, she sort of stands up for Mac, saying he is better than them, but then slips up in telling where he is.