I like the idea of the father misleading her - telling her the family's in a bit of financial difficulty and she'll have to help by earning some cash this weekend working for the rich woman around the corner, helping in her business. Round she goes, smartly dressed, thinking she'll be earning her family $100 as a photographer's model for the weekend, rather than $100k as a sadist's plaything for life - and by the time she finds out she's been tricked, she's already firmly locked up.

I think I'd open with the father worrying over money, but not mentioning the amount; later, he discusses it with a rich female acquaintance, who suggests there is a way his daughter could help him out - but let the reader discover the severity of their predicaments at the same time his daughter does.