Function to change meta value in database for each post

You’ll only need to run it once, but this should get all your posts, and then loop through them and update the meta.

add_action('admin_init', 'deleteCommaDB');

function deleteCommaDB(){

    // The Query
    $args = array ( 'posts_per_page' => -1 );
    $the_query = new WP_Query( $args );

    // The Loop
    while ( $the_query->have_posts() ) :
        $the_query->the_post();

         $price_key = 'prijs';
        $getPrice = get_post_meta( get_the_ID(), $price_key, true);
        $newPrice = str_replace(array(','), '', $getPrice);
        update_post_meta(get_the_ID(), $price_key, $newPrice);

    endwhile;

}

Your function runs only when a post is inserted. You might want to consider running on save_post and sanitizing the $_POST input in the first place.