Have you called Asiana to ask what their policy is with respect to a lap fare using miles for an infant booked in conjunction with a mileage ticket issued by United? If they will allow you to book a ticket for 10% of the miles they charge normally and you have enough SPG points to do it, that would be the cheapest. But Asiana is pretty challenging to deal with sometimes (faxes and forms and written requests). If they say yes, then you can issue the ticket for 120,000 miles on UA and then transfer miles from SPG to Asiana Club and then issue the ticket for the infant. But please allow plenty of time. 7000 or 8000 miles (plus 50% more if it's peak season) is probably a lot better than $600. But I wouldn't get my hopes up that Asiana will allow this without talking to them first.