Adding a beginner strings program - your brass players have a beginner path already, to be fair to the strings you need that for them. You can't throw a beginning violin player in with 2nd/3rd-year brass players. Include funding for cellos and basses (you don't really need them for mariachi, but if you're adding strings...) Figure out where this class (more than one?) will fit in the master schedule. Do some analysis/guessing about how this will affect beginner band numbers, and therefore high school numbers down the line.
We had a guitar class already, but you should add a beginner guitar class as well. Include funding for the "specialty" guitar-like instruments that mariachi uses. You need vihuelas (5-string soprano voice) and guitarrons (6 string bass voice, NOT the same thing as an acoustic bass guitar). Do the same guessing about effects on your band numbers.
Are you pulling singers from an existing choir program? How does the choir director feel about that? Is your choir director qualified to teach/help with Spanish diction (keep in mind Mariachi Spanish is not the same as classical choral Spanish). You don't need to be fluent in the language, but you need to know how to read and pronounce the words correctly.
I added some additional electronics stuff (we had nothing) to make sure the group could perform. Traveling PA system, good vocal mics, mic stands, cables, etc.
I also added uniforms. Our school provided nice tuxedos/dresses for band and choir, so it would only be fair to dress the mariachi in traditional uniforms (traje de charro).
I found some kind of summer workshop designed to teach mariachi to band directors, and worked up a quote to send myself to that (including travel, lodging, per diem).
Where will your mariachi group perform? If they're going to any kind of contest, there's a cost to that. If they're having any concerts in addition to what you already do, that should figure into your stipend.
When I was all done, the total amount to start up a mariachi was more than my annual band budget, and I was asking for another 1/2 position. This is why some districts have started adding a mariachi director as an additional position at the high school.