For new customers only, Checks Unlimited cuts up to 75% off select checks via coupon code "TGY7". With free shipping, that's tied with our mention from three weeks ago as the best discount we've seen from Checks Unlimited. (It typically only takes up to 70% off.)
A best bet is one box of Securiguard Parchment Checks (pictured) for $21.99 before the above coupon, $5.50 after. (Search under "Special Edition Checks" to find this style.) With free shipping, that's $16 off and the lowest total price we could find.
Note: Up to four boxes of checks can be purchased at this per-box price.