Perhaps you were being sarcastic, but in case not, don't feel like you got taken on your first day in Mexico. The guy who helped you and then hit you up for a fee probably earned his bread that day. As many who've traveled through Central & South Am. know, wealth is relative and right now you are wealthier than perhaps 99% of the folks that you're likely to meet on your trip.
When dealing with guides, taxis, or anyone offering assistance, consider what the help is worth to you and negotiate or simply offer a price beforehand. This way, you don't feel like you are being extorted or that you are cheating someone.
Have a great adventure and thanks for sharing it with us.