The DC15 is indeed much rarer than the DC14, and I currently own both of these. I bought my DC15 new in 2005, and was actually quite pleased with it. Its much better quality than the later DC24/DC25 and has a proper size brushroll and all metal soleplate. It is also quite quiet - it used a Panasonic motor encased in another enclosure inside the ball, so did not use the YDK screamer motor fitted to the DC14 and DC07. It was also the first upright to use a separate brushroll motor.
The DC15 only came in 3 variants, the yellow, white and purple models (base, allergy, and animal) and was made in just 2 years, 2005 and 2006, being very quickly replaced by the DC18, which was lighter and less bulky, but did not use a ball as such, more a thinner longer roller. People still moaned about the DC18 being too heavy, so the DC24 and DC25 replaced the DC18 fairly quickly too. DC18's in good condition are now getting rarer as well.
The DC15 was very over engineered in all honesty and far too heavy. I found it OK to use, but I am a man, and women who have tried my DC15 have said its far too heavy and awkward to use. The DC14 was tried and tested using the same technology as the DC07 which was a huge seller, it was lighter and whilst I don't think it was any easier to use than the DC15, other people saw the ball as a gimmick at the time, and I myself fell for Dyson's hype and marketing being a lot younger and more foolish than I am now. When I bought the Dc15 I sold my previous Dyson, A purple and lime DC07 to a woman from work, who loved it. I had bought the DC07 only 2 years before, but had to have the DC15 as it looked so futuristic with the ball.
The first one I had didn't work properly and I took it back to Currys for an exchange. The brushroll stayed on all the time and didn't go off when the vac was in the upright position.
The second one I got developed the same problem with the upright switch located behind the upright stand assembly, so I had Dyson send an engineer to fix it. After this it was OK, and I used it regularly with no problems again.
The price of used DC15's is higher because they are not that common and possibly because of their future collectable status.