How about $249 for Co-Motion's kit, 3rd row, last on the right?
Your best bet on used if nothing comes of this posting is to search all of Craigslist for someone selling a tandem where they are including a child stoker kit and seeing if they'd be willing to separate the kid crank from the sale of the bike....
This is a google search string that will get you close to what you're looking for, although you'll need to read the short descriptions and you'll also find a lot of these ads are no longer current:
site:craigslist.org tandem kid crank
site:craigslist.org tandem child crank
Here are two examples of 'finds'....
A Cannondale MT2000 with kiddie cranks included for under $1; this is a great price for this tandem BTW...
An older Santana Arriva with a tandem and a child kid crankset.
Similarly, you could search at http://www.tandemmag.com/classified/ using the terms 'child stoker' and see if any of these folks still have them available and/or check with some of the folks who have 'wanted to buy ads' for stoker cranks to see how they have faired.