Amazon has Baby Jogger City Select Second Seat Kit on sale for $84.99 with free shipping. It is available teal, silver and black for the same sale price.
With this second seat kit, you can convert the Baby Jogger City Select single stroller into a double stroller. The kit comes complete with the second seat along with its mounting brackets.
This is the lowest price we have seen on Amazon. According to the Amazon tracker we use, the average price is usually around $160.