Properly set up default settings

gistfile1.php

<?php

register_activation_hook( __FILE__, 'halloween_store_install' );

function halloween_store_install() {

    //setup default option values
    $default_options = array(
        'currency_sign' => '$'
    );

    $options = get_option( 'halloween_options', $default_options );

    //save our default option values
    update_option( 'halloween_options', $options );

}