Okay, here's a bigger discount. This is just a suggestion, but you can use a layered approach. You already mentioned the discount club. It costs to sign up, but it's worth it especially if you buy and sell a lot of domains each month. Next is bulk registration. Your discount gets deeper based on volume, so try to bundle your domain purchases and name variations in a single purchase. As if that wasn't enough savings already, you can start your purchase transaction at Fatwallet.com to get the 12% cash back from the Godaddy affiliate program. FW is the affiliate, and after 60 or 90 days you get the commission percentage deposited into your FW account from Godaddy. Then ask for a check or Paypal from FW.
But wait there's more. Godaddy also has a "gold" account that you can fund with advance deposits by check or money order. If you select your "gold" account as your method of payment at checkout, I think you can get another 2% discount on top of everything else. As an alternative you can also try using a rewards credit card, or the affinity card with the Godaddy brand I've seen advertised all over the Godaddy website. I think that card would give you another 5% discount. Nice.
You can search for coupon codes to enter at checkout. At any time there are many different codes for various products, so you'll have to figure out what you want to use. There might be restrictions about trying to combine coupon codes with other discounts, so check the fine print. Godaddy sends out a regular email newsletter that features special promotions and other goodies. Just keep your eyes open.
At this point, using a layered approach you can drive your costs to buy a domain name to about $6.49 per .com or less. Maybe lower if you factor in the cash back from FW and your desired method of payment. But if you find yourself involved in many hundreds of domains, you may want to consider their "super reseller" program. That way you could buy domains from yourself using the reseller account. In a sense you could factor in the other items for resale (recurring revenue, etc) as part of driving your overall expenses down.
