I guess is because you are using "es" values instead of overwritten them with the ones in es_MX

Thanks for the heads up, I'll look into the cause of the problem. You're right that it's because we're using es values, but not because we're not overwriting them with the values in es_MX. In the CLDR we're using, es_MX.xml looks like this:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE ldml SYSTEM "../../common/dtd/ldml.dtd">
<!-- Copyright © 1991-2013 Unicode, Inc.
CLDR data files are interpreted according to the LDML specification 
(http://unicode.org/reports/tr35/)
For terms of use, see http://www.unicode.org/copyright.html
-->
<ldml>
    <identity>
        <version number="$Revision: 8245 $"/>
        <generation date="$Date: 2013-02-24 22:19:26 -0600 (Sun, 24 Feb 2013) $"/>
        <language type="es"/>
        <territory type="MX"/>
    </identity>
    <numbers>
        <currencies>
            <currency type="MXN">
                <symbol>$</symbol>
            </currency>
        </currencies>
    </numbers>
</ldml>

This implies all the missing data should be inherited from es, unless I've misunderstood the multiple inheritance chain somehow. Either that or we just need to use a newer version of the CLDR.

OK, I've updated my local CLDR core and tools files to version 24 and the correct data is now being built, from the expanded data in the /common/main/es_MX.xml file.

I still need to tweak the config and the compiler a bit to minimise the output on the JSON files, which I'll try and get done ASAP before I push the changes.

Fixed by 47786a1. Thanks again.