For a true Buy One Get One Free this calculation should work:
abs([offerID] == '1' && [Quantity] >= '2')?([Price] * floor([Quantity] / 2)):0
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.