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add_filter( 'woocommerce_subscriptions_product_sign_up_fee', 'handsome_bearded_guy_filter_sign_up_fee', 10, 2 );

function handsome_bearded_guy_filter_sign_up_fee( $subscription_sign_up_fee, $product ) {
	//this will only work if the necessary object and method are present
	if ( class_exists( 'WC_Memberships_Member_Discounts' ) && method_exists( 'WC_Memberships_Member_Discounts', 'get_discounted_price' ) ) {
		$discounter = new WC_Memberships_Member_Discounts;
		//taken from WooCommerce Memberships
		/**
			 * Get product discounted price for member
			 *
			 * @since 1.3.0
			 * @param int $base_price Original price
			 * @param int|WC_product $product Product ID or product object
			 * @param int $user_id Optional. Defaults to current user id.
			 * @return int|null discounted price, or null if no discount applies
			 */
		$discounted_fee = $discounter->get_discounted_price( $subscription_sign_up_fee, $product, null );
		//see above - check here for null
		//because if null, no discount applies but if not null, then set subscription sign up fee
		//value to the discounted fee
		$subscription_sign_up_fee = ( ! is_null( $discounted_fee ) ) ? $discounted_fee : $subscription_sign_up_fee;
	}
	return $subscription_sign_up_fee;
}