I'm sure some of you will of probably seen this, but there's currently a BT/ISO 15 ATC over on kickstarter - CNC BT15/ISO15 ATC Spindle Cartridge and Spindle Shaft Kit by Dale Walsh — Kickstarter

Looks quite promising, and I am very tempted for future use.
There are various options ranging from $10 just for the plans, to $2500 for a full spindle, air cylinder and 30 tool holders.
A complete spindle on it's own is $775, although it's not to clear on what's included with that option.