Does the infant need to have a seat. Travel as a lap child and the cost would be around $250

If needing a seat, then rather than use miles for one of the adults, purchase 1 x r/t $800 adult ticket plus 1 x approx $80 r/t infant ticket for a total of about $880
or , of course purchase the miles needed at $764.39 - but I would suggest that buying ticket rather than miles would be much better value