This is an old thread but just to say: It is a very fine song (especially the Jane Turriff version): the crooked sixpence is a "broken token" (as occurs in quite a few scottish songs/ballads) and has nothing to do with prostitution. In other words the two protaganists knew each other and the maid gave him a token to remember her by, and with which to identify himself when next they met. There are a few (improbable but beguiling) scots songs in which the female apparently does not recognise her lover, he tests her, she passes the test, and he then brings out the broken token ("broken" token because the token is often broken in to two, one half for each).
jrc