@using System.Globalization
@model decimal?
@{
    Layout = "_FieldLayout.cshtml";
}
<div class="input-group">
    <span class="input-group-append">
        &euro;@Html.TextBox(
                  "",
                  Model == null 
                      ? "" 
                      : string.Format(CultureInfo.CurrentCulture, "{0:F2}", ViewData.ModelMetadata.Model))
    </span>
</div>