Unable to process php via shortcode

The problem is that you are using file_get_contents() which only gets the content of the file (hence the name)

if you need to render(evaluate and process the php file) then just use include().

But also if your cup_bc_layout.php file echo’s out something then you should put the include inside an object buffer and then return the value to make sure the shortcode works properly.

So try this:

function cup_bc_shortcode($atts,$content=NULL){
    ob_start();
    include('/html/cup_bc_layout.php');
    $return_val = ob_get_contents();
    ob_end_clean();     
    return $return_val;
}
add_shortcode('budgetcalc', 'cup_bc_shortcode');