角度数据 - 无法将更新值绑定到范围（使用滑块）

I'm trying to use Angular to create a tool that accepts user input from a slider tool, and automatically updates an "estimate" field whenever the values are changed. However, the data only seems to be binding on the way to the view; the user input is displayed, but doesn't seem to register in the scope. I'm trying to use Angular to create a tool that