There is no "departure" fee, but there is a "DNI"/tourist fee that some visitors must pay before leaving Mexico -- I think this is the fee to which you are referring.
This is from the website of the Mexican Embassy in Belize...
Payment of the DNI Fee
A fee known as Derecho de No Migrante (DNI) or Non-Immigrant Right is associated with the FMM, and in some circumstances it must be paid before leaving Mexico.
All foreign nationals who enter Mexico must pay the DNI fee in three different cases:
1. Visitors who enter Mexico and stay more than seven consecutive days.
2. Visitors who enter Mexico and transit through to a third country regardless of the number of days in Mexico.
3. Visitors who enter Mexico with a Forma Migratoria de Visitante Local (FMVL) border card and stay more than three consecutive days.
Visitors who are exempt from paying the fee include:
1. Visitors who enter Mexico, stay for seven days or less, and return to the country of origin.
2. FMVL border card holders who stay less than 3 days.
3. FM3 and FM2 holders.
Here are two other points from the website:
3. For entries by land or sea of tourists, intransit travelers and businesspersons, when there are no banking institutions or when it is not during regular working hours or days, tha payment of fees is made directly to the immigration authority who should issue the form known as "Comprobante" (a receipt) which is proof of payment.
6. The payments does not have to be done exclusively at BANJÉRCITO but it is the only banking institution that has a branch at the points of entry borders.
The last time I heard about the amount paid at the land border, it was about 360 pesos (over 6 months ago). Whenever the fee has been included with an airline fee, the amount has always been the equivalent of US$22 to $26. I've never seen an amount quoted on any official Mexican website.