String
simply does not implement IFormattable
. To use the formatting, remove .ToString() so that you aren't passing in a String.
<%# String.Format("{0:f2}",DataBinder.Eval(Container.DataItem, "pricerange"))%>
To see this more explicitly, run this code:
Console.WriteLine(string.Format("{0:f2}", "123.888"));
Console.WriteLine(string.Format("{0:f2}", 123.888));
which outputs
123.888
123.89
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…