One can find this information in their fare rules.

IF A DECREASE OCCURS AFTER A TICKET IS PURCHASED AND PRIOR TO ANY TRAVEL ON THE TICKET OR A NEW FARE FOR WHICH THE PASSENGER QUALIFIES BECOMES EFFECTIVE THE DIFFERENCE IN FARE MAY BE CREDITED. FOR COMPLETE DETAILS SEE PARAGRAPH VI BELOW

IF A DECREASE OCCURS AFTER A TICKET IS PURCHASED AND PRIOR TO TRAVEL ON THE TICKET OR A NEW FARE FOR WHICH THE PASSENGER QUALIFIES BECOMES EFFECTIVE THE DIFFERENCE IN FARES WILL BE CREDITED PROVIDED . 1. THERE ARE NO CHANGES TO ORIGIN/DESTINATION/ STOPOVER POINTS/FLIGHTS/DATES. . 2. ALL CONDITIONS OF THE NEW/REDUCED FARE MUST BE MET. THE ORIGINAL TICKET DATE OF ISSUE MAY NOT BE USED TO SATISFY THE ADVANCE RESERVATION/ TICKETING REQUIREMENTS. THE BOOKING CODE OF THE NEW/REDUCED FARE MAY DIFFER FROM THE BOOKING CODE ON THE ORIGINAL TICKET. . THE PASSENGER WILL RECEIVE A NONREFUNDABLE MCO LESS A USD50.00 ADMINISTRATIVE SERVICE FEE.

The fee is therefore $50; not $100. Check your fare rules. The above quoted text was from a the MR LAX-PHX fare that I just pulled up on nwa.com