On the last day of the last year, U.S. Department of State has published a final rule on fee changes. Annotation to th rule explained that the fees were changed to reduce the burden on Mexican citizens under 15 years of age applying for a Boarder Crossing card as well as to comply with the newly enacted Emergency Afghan Allies Extension Act of 2014:
The Department of State amends the Schedule of Fees for Consular Services (Schedule) for visa fees. More specifically, the rule amends the Border Crossing Card fee paid by a Mexican citizen under age 15 whose parent or guardian has applied or is applying for a border crossing card (the “reduced Border Crossing Card fee”). The Department of State is increasing the fee in light of the passage of the Emergency Afghan Allies Extension Act of 2014, which added a $1 surcharge to the fees for Machine Readable Visa (MRV) and Border Crossing Card (BCC) application processing. The Department must raise the reduced Border Crossing Card fee by $1, for a total fee of $17, to continue to collect the legislatively mandated fee amount of $13 and all applicable surcharges.
Reference: Federal Register / Vol. 79, No. 250 / Wednesday, December 31, 2014 / Rules and Regulations [PDF version]