Arduino mega - best replacement for atmega 1280
The ATMega1280 and ATMega2560 are pin for pin equivalents, with what I can tell the only difference is the ATMega2560 has twice the flash size of the ATMega1280. You should be able to do a drop-in replacement without any problems, in theory. Newer Arduino Megas use the AtMega2560 chip, you can try looking at the schematics to see if there are any significant differences which would suggest the two are not interchangeable. The only possible downside would be cost.

Before attempting this, though, I would check to make sure everything else on the Arduino board is working, and that the original ATMega chip is indeed bricked beyond repair. As you said, it's not a particularly cheap chip so it would be counter-productive to plug in a new chip to find the board still doesn't work.