The cost for extra pages is $ 82 . It is added in packets of 24 pages with max of 3 packets so make sure to ask if they can add 3 packets (same price $ 82). I know embassy in SIN does it in a day or two but will only add 2 packets.
I just read that US gov. is getting rid of this service by Dec.31, 2015!
If you are near PHL you can get a new passport on the same day at the Passport agency for extra $ 60, just need an appointment and remember to request the 52 pages version. I don't know about other locations.
My Indian friends tell me they just rubber band old and new passport together when they travel on visas. Hope that is possible especially when one has $ 150 visa valid for 7 more years in the old passport.