I voted in the poll, but I would like to share my suggestion to see what people think:
Separate the kits. For add-on, commit to buying 500. That's 500 x $4 = $2,000. Then present the prices in tiers. Set our own MOQ for group buy purposes, say 200 to keep price reasonable and give people assurance they are not committing to buy unless price reaches $10. So prices would look like this:
Order qty Price per order
200 $10
300 $6.67
400 $5
500 $4
Obviously, for any quantity below 500, there would be more units produced than orders, and we could just divide up the extras evenly, have a lottery to see who gets extra sets, or sell them later and donate proceeds to GH. If we got 250 orders, everyone could get two sets or do whatever else with the extras. My guess is that people who want the add-ons wouldn't mind paying slightly more. I mean, come on, these add-on prices even at the 200 order mark are still way below what similar SP sets generally cost.