Welcome to FT
Here is the official NW explanation....
http://www.nwa.com/worldperks/travel/flex.shtml
You have more chance with a Mon-Thur travel - but in general it can be a crap shoot. Sometimes it is a matter of trying months in advance.
In your place - if I have the money - paying $700 all in is not a bad price and you will earn many more miles.