A quick back of the envelope calculation is as follows:
$80 for 85 watt charger on amazon [0] + $3 * 2 for Sugru on both ends of the cable [1] + $5.95 for USPS priority 3 day shipping [2] ~= $92
With this supply chain (if it can be called that), you can still estimate shipping the charger to the end consumer within 6 days (2 days with Prime shipping with Amazon + 3 days for USPS). I did not factor in the Prime membership and the mold in the costs though, but it can be factored in fairly easily. Also, I chose to use USPS shipping costs, naturally your mileage will vary because your company/business probably has higher costs (cool service by the way, and congrats :))
[0] 85 W charger: http://amzn.com/B004L6MLPQ [1] Sugru 8 pack: http://amzn.com/B00EU7DBNM [2] Small flat rate box from USPS: http://pe.usps.com/text/dmm300/Notice123.htm#2517977
I also Sugru mine like this on both ends of the cable: http://d1u2s20mo6at4b.cloudfront.net/wp-content/uploads/appl...
I'm considering making a mold so the Sugru looks nice when done and selling robustified chargers on Shopify. I already have a shipping business (MonthlyBoxer.com) so that part is easy. Thinking perhaps about offering a discount for returning the broken charger to us for refurbishing, so we can offer both new and refurb chargers.
Anecdotally the Sugru doubles the life of the charger cables (and makes your charger much more identifiable). I'd need to charge $99 to make it work, but it's fairly cheap to throw together a site and reply to the myriad forum posts saying "where can I buy a better Macbook charger" with a link to gauge interest.