[asp.net-mvc] Posting form to different MVC post action depending on the clicked submit button

This sounds to me like what you have is one command with 2 outputs, I would opt for making the change in both client and server for this.

At the client, use JS to build up the URL you want to post to (use JQuery for simplicity) i.e.

<script type="text/javascript">
    $(function() {
        // this code detects a button click and sets an `option` attribute
        // in the form to be the `name` attribute of whichever button was clicked
        $('form input[type=submit]').click(function() {
            var $form = $('form');
            form.removeAttr('option');
            form.attr('option', $(this).attr('name'));
        });
        // this code updates the URL before the form is submitted
        $("form").submit(function(e) { 
            var option = $(this).attr("option");
            if (option) {
                e.preventDefault();
                var currentUrl = $(this).attr("action");
                $(this).attr('action', currentUrl + "/" + option).submit();     
            }
        });
    });
</script>
...
<input type="submit" ... />
<input type="submit" name="excel" ... />

Now at the server side we can add a new route to handle the excel request

routes.MapRoute(
    name: "ExcelExport",
    url: "SearchDisplay/Submit/excel",
    defaults: new
    {
        controller = "SearchDisplay",
        action = "SubmitExcel",
    });

You can setup 2 distinct actions

public ActionResult SubmitExcel(SearchCostPage model)
{
   ...
}

public ActionResult Submit(SearchCostPage model)
{
   ...
}

Or you can use the ActionName attribute as an alias

public ActionResult Submit(SearchCostPage model)
{
   ...
}

[ActionName("SubmitExcel")]
public ActionResult Submit(SearchCostPage model)
{
   ...
}