Transilien (the name of Paris’s intercity transport) has a great offer going. It’s called Forfait Loisirs. Basically, the French rail service (Transilien) will sell you a combination transport and museum admission ticket for the Versailles Palace. This means that you can buy your entrance ticket in advance, avoid long lines and get your transit and entrance fee taken care of at the same time.
Judging from the price of most minivan or coach tours (usually over 70 Euros per person), the total price of 21.65 Euros on weekdays or 25.95 Euros on weekends looks pretty good. Keep in mind when you check out the Transilien site that you need to confirm the availability for specific dates. Download the calendar which marks off available dates in green or orange depending on whether you buy weekend of weekday tickets.
>> More info on Forfait Loisirs