If you have an older movie, shot with older media on a tight budget, chances are your blu-ray is not gonna be much better than a DVD, esp. if the publisher doesn't spend much on remastering.
Gotta love those who buy Casablanca in Blu-Ray for example. Lawrence of Arabia on the other hand was shot in 70mm, so it could be great in bluray. Kubrick probably also had the cash to spend on media and his movies are in the $20 range.
A good objective test of worthwhile Blu-Ray vs. DVD is a freeze on a screenshot with written text (a letter or a page in a book). Typically, you can easily read the Blu-Ray but DVD is blurry at best.
My favorite in terms of image quality is Apocalypto. Maybe not the best overall, but the first one that REALLY wowed me.
Last but not least the price premium has been dropping steeply. I wait and get most of my BluRays for less than 20$ CAD, often for $10. Mind you I am also picking up DVDs for $6 as well (hello music industry - when you gonna lower your prices???).