You aren't doing anything wrong. Number fields off all types remove placeholders like this behind the decimal in the database. You can simply add it back and format it to two decimals when you display it.
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.