Widget Mini Calendar…
Thanks for posting. I was able to find the string in the Spanish translation of The Events Calendar:
#: ../public/template-tags/general.php:531 msgid "Free" msgstr "Gratuito"
You can change that using a translation program like PoEdit. Change the translation from Gratuito to whatever word you prefer, and then save the .po file. Saving the file will also generate a new matching .mo file. Drop those two files into the /lang folder for The Events Calendar (replacing the existing Spanish translation) and you should see the word change in your site.
You’ll notice that this also changes the word on your single event pages and any other widgets. The widgets pull the cost (and any associated translation) from the single event page, so changing one changes the other.
If you have any trouble, let me know and I’ll do my best to help out.