cost.js

/**
 * https://tfl.gov.uk/corporate/terms-and-conditions/tfl-call-charges
 *
 * > The rate is 0.66p per minute.
 * > There is also a 40p connection charge.
 * > Call charges are rounded up to the nearest 10p, with a minimum charge of 60p.
 *
 * @param {number} seconds
 * @return {string} Cost in GBP
 */
function cost(seconds) {
	// Round up per minute
	var minutes = Math.ceil(seconds / 60);
	// Calling charge of 0.66p per minute
	// Connection charge of 40p per call
	// Minimum total charge of 60p per call
	var charge = Math.ceil(Math.max(60, 40 + (minutes * 0.66)));
	// Round up to nearest 10p
	charge = Math.ceil(charge / 10) * 10;
	// 123p -> £ 1.23
	return '£ ' + (charge / 100).toFixed(2);
}