Just so there is an apples to apples comparison, pjrc does sell the Teensy 2.0 without headers for $16. CONUS shipping for one is ~$3, going up to ~$3.5 for eight. So while you definitely do save with this, the savings are not quite as pronounced as the original post indicates.
$20.60 for one from pjrc; $16.50 from this group buy
$145 for eight from pjrc; $118 for eight from this group buy
Definite thanks to jdcarpe for organizing this, though!
Edit: Missed the headers, updating comparison. This (with headers) is still cheaper than buying from pjrc without headers, by the way.