load external file in plugin

Hi,
I’ve written a very simple plugin to replace a variable with a price:

function the_price_thing($cursuscode){
$cursuscode = str_replace('TAANNEM-prijs', '339 €', $cursuscode);
return $cursuscode;
}

add_filter('the_content', 'the_price_thing');

The list is a lot longer and is used in several websites.
Now I would like to use the same function, but place the list on an external domain, so I only have to update it once. I’ver tried different ways to include the txt or php file into the function, but nothing works for me.

All suggestions welcome!!!