Upper Reward and Lap Infant

Hi All,
I have found 2 Gs return LHR - LAX in March, 200k miles + £1127 for 2 adults. Thats the good news.
The bad news is that I have also been quoted £479 to add a lap infant to the booking. Does this seem excessive?
My understanding of a lap infant fare was something like 10% of adult base fare plus taxes. Upper return for our dates for 1 adult currently looking at about £2000, so I can't figure out where £479 is coming from.
Thanks!
I have found 2 Gs return LHR - LAX in March, 200k miles + £1127 for 2 adults. Thats the good news.
The bad news is that I have also been quoted £479 to add a lap infant to the booking. Does this seem excessive?
My understanding of a lap infant fare was something like 10% of adult base fare plus taxes. Upper return for our dates for 1 adult currently looking at about £2000, so I can't figure out where £479 is coming from.
Thanks!