I've looked into this as well, I found a load of options on another board and they were all working out around 100 or there about for 4 of us (kids 3 while we're there and 4). I also looked at Easyjet which was the most expensive shuttle.
The TGV seemed to be about 87 return but that was based on travelling in April, we go in May. You can only book ahead 3 months (explained at the side panel) but I missed this and got the helpful message of a triangle with an exclamation mark and a "." as my error....I retried with April and got these prices. This was based on 3 of us as you can choose to sit a child under 4 on your lap and not pay for them - I think for a 10 minute train journey, that's fine!
You have to book a time for the TGV which might push the price up for us as the evening trains seemed to cost slightly more and Easy Jet get us in at 18:30. This price was for a non-flexible ticket, 10 on the day to chnage the booking. There's also then the problem of getting from the station to Santa Fe but I think we should be ok for a transfer bus for the hotel.
When booking it, the website had a lot of difficulty with "MARNE LA VALLEE CHESSY" offering me 4 options - I chose MARNE LA VALLEE CHESSY - SNCF then it told me this didn't exist and offered me MARNE LA VALLEE CHESSY. The airport was fine coming up as Roissy - CDG.
I have to say, it's not the best website to use.