Call in, and talk to retentions.. and see what they can do.
Other than that.. your stuck. Around 90% of the $ of that roaming charge, is not from rogers, it is what the mexico telecom is charging rogers, for you going on their network... rogers just passes that back along to you (plus their 10%ish)
This is why if you travel, you either need to add on a specific travel package first... or if only going to be using in wifi only areas, MUST make sure the phone is off otherwise, or at least that roaming is turned off.
(I know the above, sounds kinda harsh.. i dont intend it that way... but thats the way roaming works. ALL other carriers, their roaming pretty much works the exact same way (maybe slight $ value differences, but you would be charged the same way))