you have the trigger set correctly now, but not the calculation.
for the calculation, use Based on items with a specific value:
Total of 'TotalPrice' where 'MyColumn' has a value of '1' 'times' '0.5'
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.