look at it this way, if an airline advertised a return fare from SYD to LAX for $1142(per adult). Kids are very roughly 75%, so 2a + 2c = $3997. Bet they would get many more takers if they advertised $999 (for everyone) + the power of price points is enormous. Having worked in retail for 25 years, I can tell you, $999 is about $200 or more, less than $1000, in the mind.
Maybe a good way to move seats, is to say ...
1 seat (booked at same time, same itinerary) $999
2, $979 each
3, $959 each
4, $939 each
5, $919 each
6, $899 each
or instead of dropping price, give something away, like extra ff pts, increasing for each person the more in booking.
or offer the 10th seat free. It's amazing how one person, can get 9 others to go, if he/she is getting a free ticket.