It sounds like they owe you 25% of your admittedly cheap fare and maybe some meal vouchers. From section 7 of the Aeromexico contract of carriage:If, due to causes attributable to the Carrier, boarding is denied to any Passenger or due to the issuance of Tickets in excess of the capacity available on the aircraft, in terms of section VII of article 38 of the Regulations of the Law of Civil Aviation, the Carrier shall be responsible for informing Passengers of the following options available to them under article 52 of the Law of Civil Aviation:

• That the Carrier reimburses to Passengers the price of the Ticket or the corresponding portion corresponding to the part of the trip not covered. In this case the Carrier shall also cover an indemnity to the affected Passenger which shall not be lower than 25 percent of the price of the Ticket or of the part of the trip not performed;
  
• That the Carrier offers to the Passenger, with all means available to it, replacement Transportation on the first available flight providing, as a minimum and without charge, telephone or cable communication services to the place of destination; meals in accordance with the delay time until the boarding of the next flight; a room at an airport hotel or in town when overnight stay is required and, in this case, ground transportation from and to the airports, or
  
• That the Carrier transports Passengers on a later date convenient to the Passenger towards the destination for which boarding was denied, in this case the Carrier shall also cover an indemnity to the affected Passenger which shall not be lower than 25 percent of the price of the Ticket or of the portion of the trip not covered.