I will admit that I was disappointed with the Flowers in the Attic movie, but I loved Christopher Young's soundtrack. It is the perfect accompaniment for the dark, gothic style that the filmmakers used for the movie. The combination of the orchestra with the haunting, eerie vocals is very evocative. It makes you feel how these children would have felt. The movie came out when I was fairly young - I remember the soundtrack actually being scary to me. Today I listen to it and can hear its brilliance and beauty.
The reason people sell it for so much is because it's been out of print for a long time and is incredibly difficult to find. I once owned a copy but ended up selling it (for exactly the same price I paid, which was $70, I believe). It's rather sad that they haven't released it again.