I was able to replicate this. It's due to the column types you're trying to subtract.
I created a RadGridView
and populated it using this code, which compiles fine but displays the exact error you received when it tries to display the grid at runtime.
radGridView1.Columns.Add(new GridViewTextBoxColumn("age"));
radGridView1.Columns.Add(new GridViewTextBoxColumn("iq"));
radGridView1.Columns.Add(new GridViewDecimalColumn("total"));
radGridView1.Rows.Add("5", "5");
radGridView1.Columns["total"].Expression = "age - iq";
Changed the column types accordingly and it worked fine:
radGridView1.Columns.Add(new GridViewDecimalColumn("age"));
radGridView1.Columns.Add(new GridViewDecimalColumn("iq"));
You just need to make sure the column data are numbers, not strings.