"a thirteen-year-old girl for a few pounds"

Thirteen was the age of consent in the Victorian period, and having sex with a girl younger than this age carried much more severe penalties.  In 1897, almost 50% of a sample of 100 "fallen" women and girls given shelter by the Salvation Army were under 16 and almost 75% under 18 (source).  Eye-witness accounts of child prositution and the solicitation of children can be found here.