I'll try an analogy:
I buy a lot of old VHS videos, usually from charity shops but occasionally from specialist dealers. A year or two ago the charity shops would be charging (typically) �1.99 for a video, whereas the dealers would be charging (typically) �3.99 for a 'standard' video or �5.99 for a rare tape.
Nowadays, most charity shops charge 49p for a video (or even 5 for a �1). Many secondhand video dealers have ceased selling 'standard' VHS videos. Those that do are selling them off for 99p. Rare VHS tapes (such as the first series of 'Last of the Summer Wine' or 'world cinema' tapes) still command prices of �2.99 or �3.99.
I see no reason why 'normal' (i.e. pre-BluRay) DVDs won't follow the same pattern of prices. Although your Disney DVDs are 'popular', they're not 'rare'. (Even if they were, using my analogy suggests that the price might still fall). Their value is far more likely to go down than up.
Chris