I think it is $125 USD...but what sort of re-route are you doing? Change of continent may affect the tkt price...This is what the fare rules say:

Rebooking/Rerouting
Prior to departure
Unlimited changes permitted without charge
After departure
Date/time changes permitted at no charge. Routing changes other than to date/time permitted at a charge of USD125.00 per
transaction.
If the rerouting results in a change to the number of continents or extra flight segments charged, standard rerouting procedures
apply. The ticket shall be recalculated based on the new itinerary. Tickets may be reissued to/from a 3, 4, 5 or 6 continent journey.
Any increase in fare will be collected and any decrease will be refunded in the form of an MCO for refund in the country of sale.
Any applicable change fee penalty will apply.
In the case of certified death/illness of the passenger or passenger’s immediate family member or accompanying passenger
voluntary rerouting is permitted free.

I used the tkt desk at LHR T4, and the guy there was fantastic! Even converted a paper ticket into e-ticket!

Good luck.