To go to Versailles-Château-Rive-Gauche from the center, you only need zones 1-4. A Navigo semaine for zones 1-4 is 32 € or 11.60 € more than a zones 1-2 pass. According to [the map on Wikipedia](http://upload.wikimedia.org/wikipedia/commons/b/b4/RER_C.svg), the last station in zone 2 on RER C trains to Versailles is Issy-Val-de-Seine.

A ticket from this station to the Château costs 1.80 € and this should also be the price of a “complément de parcours” if you have a Navigo for zones 1-2. A round trip would therefore cost you 3.60 €. Even twice the full fare from the center (3.35 € times two so 6.70 €) is still under the difference between the Navigo semaine zones 1-2 and 1-4 prices.

Sources used:

* [Navigo prices](http://www.transilien.com/static/tarifs/forfait-navigo-semaine-mois) on transilien.com
* [Fare finder](http://www.transilien.com/tarif/tarif/init) on transilien.com
* [How “complément de parcours” prices are computed](http://www.ratp.fr/fr/ratp/r_84552/complement-de-parcours/) on ratp.fr