Let'stake the 50k per day example and do so calculating..
50 * 24 = 1.200.000
This is your profit per game month. Now say you need to wait three months for the plane to be delivered. That makes the missed profit 3.6 million. If you could have gotten one at 700k over value (making it 20 milion), you missed out on 2.9 milion of profit. Ofcourse, if you can get your planes for a serious discount, this changes the picture, but for a discount of that much, you need to place like 20 or 30 aircraft orders..
And that is part of the point for the larger players. Although the first planes of an order may not take9 months, if you order more than two, they are limited to 2 a day. To take your theoretical 20 aircraft, it will take at least 10 days before you get your next aircraft. So if you can afford it, you buy both on the open market and from the Q.
That said, if I was a smaller player (I was once), and had enough to buy one on the open market, I would buy 2 by either choosing another aircraft almost as good with no order delay, or order my aircraft and take the slight delay.
Last round with unlimited deliveries, there were only a few aircraft in play. This round, there are a lot of Saabs and ATR 72's, and EMB's that didn't get used. I substituted ATR 72's for my Q400's early this game, as long as my routes were about 500 miles or less, there was no noticeable difference. Certainly not enough to make up for the delivery delay.