At that age I wouldnt do long haul with a kid on my lap, not even to save $1400. We have flown long haul with kids at different ages and have done toddlers at 12mths, 14 mths, 18 mths and each time we have bitten the bullet and purchased a seat - best thing we could ever do.

They move so much and as it is there is barely any room for an adult. The sky cots have a weight restriction of I think 11 kilos so that is something else to think about and my DD2 who was 12 mths when we did long haul last year lengthwise was struggling in it, so a larger baby would not be comfortable at all. Just my thoughts tho.