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, 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 on transilien.com
- Fare finder on transilien.com
- How “complément de parcours” prices are computed on ratp.fr