public ContentResult RevertToDefault(int id, string default)
{
  /** Update the DB **/

  return Content(default);
}

<%= Ajax.ActionLink(
  "Revert",
  "RevertToDefault",
  new { id = Model.MyObject.ID, default = Model.MyObject.DefaultValue },
  new AjaxOptions { OnComplete = "function(r) { alert(r); }" })%>

<%= Ajax.ActionLink(
  "Revert",
  "RevertToDefault",
  new { id = Model.MyObject.ID, default = Model.MyObject.DefaultValue },
  new AjaxOptions { OnComplete = "function(r) { alert(r.get_data()); }" })%>