So you have 500MB a month, that will be fine if a lot of the time you connect to wifi (home, in shops/cafes/work etc.). You will only use the data when not on wifi.
As has been established, to prevent using data without meaning to, you can turn off your data and turn wifi on. Further, poke around in the Settings of your phone and you can usually either set a limit on your data or get a notification to warn you based on a threshold you set. If you speak French, you can look at this manual (not sure why there is no easily available English version...)
https://assistance.orange.fr/medias/woopic/files/content/download/731204/14648465/version/1/file/orange_rise_31_guide_utilisateur.pdf
You can find out how much you have used in WhatApp via the app itself - see
http://technology.onehowto.com/article/how-to-know-how-many-megabytes-whatsapp-uses-437.html It will tell you how many messages and how much data used, so you can get a guesstimate of data per message.
Also, bear in mind that any photo/videos sent/received in WhatsApp will increase the data usage, but text only messages will be smaller. Again, in the settings in WhatsApp you can set it so that attachments are only downloaded when on wifi (in fact you can make WhatsApp only send/receive when on wifi).
Hope that helps