The majority of assistants will not have to pay a fee in order to get their visa validated by the OFII after arrival in France. However, assistants who are also students, and who obtained student visas in their home countries, will have to pay a fee of 55€. Don't forget to get your passport stamped at the airport when you arrive in France! You will also need a copy of this stamp when you do the validation, to prove your date of entry into France.
Summary of visa validation fees for different statuses:
340€ for visiteur and conjoint de français
70€ for salarié
55€ for étudiant
free for travailleur temporaire (this includes assistants de langue)
If you are able to renew your visa/change your visa into a Carte de Séjour for the second year, then the renewal fee is 110€.