I had to extend my dates too few years ago. I bought my ticket from some nepali travel agency and my airline was Gulf Air. But i went to the Gulf Air office nearby jai nepal. They charged me $200 which i guess was their standard charge.

These travel acengy are here to just rip you off because they know you have no other options and you will have to pay. So, what i would suggest to everyone is that once you buy your ticket from them then talk with your primary airline for everything else. 

And most importanly, today someone just started this thread which I believe we should all consider. 

http://sajha.com/sajha/html/index.cfm?threadid=105063