While I admire where this has gone and found some of the conversation enlightening, the chances of anyone else getting a working spindle at a reasonable price doesn't look possible and it's a shame becuase I think you've really hit on somehting good.
I kind of consider myself a spindle aficionado, I have the luxury of working with a spindle expert who does this as part of his daily routine and I've gotten pretty good at it.
Here is some of my recent work on ATC spindles, what I started last week is an advanced version of the BT15 spindle cartridge with the disc springs internal and should be done in a couple of weeks.
Yes, even a sherline BT15 ATC conversion happened in the process which consists of a shaft, petal-clamp, disc springs and drawbar utilizing timing belt/pulleys and has sufficient tool clamping force to prevent slipping at 382lbs of pulling force.
As you can see in one of the pictures, they are real petal-clamps, nitride coated for strength and durability (over-kill, I found out).
I'm fighting with some Delta VFD's at the moment (I can't figure out how to make them do orientation) but as soon as I'm done I'd like to take what I've collected here and come up with a BT30 spindle cartridge that people can buy if we can figure out what a reasonable/fair price is I should be able to have something to purchase in about 30 days.
I do have a spindle expert at my disposal and I've made several different designs myself as well as a couple of BT30 spindles that were expensive but I believe I can produce something at a reasonable price based on the cumulative experiences and knowledge.