As tkey75 noted, $550 is pretty good for F, but these days, 40K is perfectly acceptable. In fact, it's even better because you got F - it sounds like you may not be a Delta frequent flyer, so if you were to buy coach by miles or cash, you would also have to pay for your checked luggage, which is another $25 per person per segment, in F you save $50 each and get the higher weight limit of 70lb, too. Now, 70K miles is stupid, and if you cancel to change the times, you should pay the $550 and collect the miles. However, if you are outside the 24 hr cancellation window, changes are $150, except for DMs and PMs, who can redeposit their miles for free.