In terms of the time of year to visit, as you might expect, school holidays, the Christmas period and the summer months are all popular times to travel to Paris. If your plans permit and you can possibly avoid these times then you are likely to save money. Of course there are times when it is impossible to avoid the peak season but there are still ways to ensure that you get the best possible deal for your money.
For example, days of the week are an important factor that impact on the price of flights, hotels and holiday packages. Understandably, most people want to maximise their precious holiday time and consequently weekends are by far the most popular and most expensive time to travel. Weekends in this sense generally includes Fridays and Mondays too. If your dates are flexible and if cost is a more important factor, try looking at travelling out during the midweek period and returning on a Friday or Saturday (avoid Sundays). In general, the flights and train costs will be significantly lower.