ATMEGA and Arduino Mega is a bit confusing, not your fault. I asked just to be clear ;).
I just took a look at my Arduino Mega, seems like a really hard task to replace the ATMEGA1280. I hope you will find someone who can do that for you :).
You might want to try Forum - Circuits Online. That's the biggest electronics forum in The Netherlands. Maybe someone there has the knowledge to replace the chip for you. Tweakers.net also has an electronics forum but I think requesting things like that is not allowed there (maybe in the "EL kroeg topic").
Good luck! 
Btw; where did you get the atmega1280 for just ?10-15? It's ?23.24 at Farnell (excluding VAT).