I understand you about the RER... it is not the easiest way to join Paris/CDG if you travel heavy.
About 350, consult this timetable. On the first page, the one you are interested in, it indicates the journey from Gare de l'Est to T2A/CDG is about 1h15. To be careful about the traffic jam, you should not have as you are leaving Paris, I would catch the 7h54 bus.
It is even cheaper than Roissybus or RER as it is about 4€, three metro tickets.

Hope it helps.