I think it's because the Dream Beauties are less common than the regular ponies. I've only found one, and it was at a thrift store. I bought her for $3.00, cleaned het up (and boy did shr clran up beautifully!), and sold her for $35.00.
As for me, there have been two recently that went for alot more than I expected. The first was Sweet Pocket. I expected her to go for at thr most $25.00, and i felt that was pushing it value wise. She went fr $42.00.
The second one was TAF Munchy. Ok, I know she goes for quite high, but she had a lot of nail polish on her, so I figured maybe she wouldn't go for as much as usual. Nope, she went for $38.00. I was really disapointed with that one because she is a grail pony for me, and I haf hoped to get her at a lower price (flaws and all).