I believe the huge price difference is not that the ATC spindles are very expensive, but because of how cheap the standard spindles are. I've seen 2.2Kw spindles as low as $250.
Compare a non ATC chinese spindle vs a similar power HSD. The chinese spindle is 5-10 times cheaper. This is most likely due to the huge volumes of these being produced. There are numerous sellers on Ebay, from what appears to be many different manufacturers. I'd imagine that ATC spindles are produced in much smaller quantities. In the 5 years or so that the chinese spindles have been available, I've seen one person purchase an ATC spindle, while hundreds have purchased the standard chinese spindles.
If you look at a standard and ATC spindle from a manufacturer like HSD, you'll see that the ATC sindle is 2-3 times more expensive.
A chinese ATC spindle is probably 30%-60% cheaper than an HSD or Colombo ATC spindle.