I too really enjoyed this movie!
My personal take on Pippa and Herb, is that she liked him; as she said; not necessarily for his money, but because he simply made her feel special. After suffering through all that family dysfunction, I think Pippa was just looking for someone to make her feel safe - she wasn't the most emotionally-stable character, as the movie illustrates.
The age difference is supposed to be around 20 to 30 years, I think. Adult Pippa reflects on her childhood in a 'montage' kind of approach, but it's stated that "when she was 16" she finally figured out her mother had a drug problem. I figured she moved out around age 16, lived with her aunt (and Cat) for a year or so, and then got kicked out of their place around age 18...which would make sense if Pippa dabbled in adult films for a bit, after those weird photo shoots with Cat. That puts her in the age range of about 18 to 20, when she first meets Herb; and I'd guess he's about 50 when she first talks with him at that party.
My opinion! I didn't really know what to expect from this movie, but it turned out pleasantly enough. :)