Either go by train or rent a car.
Can't think of any other way of doing it
Train timetables and fares at voyages-sncf.com
For historic weather data try myforecast.com or wunderground.com
how much is a taxi gonna cost?
Several hundred €uros.
Look at it this way. A taxi from CDG airport to the center of Paris, about 30 km, costs ~50€. A taxi from Paris to Beauvais airport, about 70 km. costs 125€-150€. Le Havre is over 200 km from Paris and you would probably have to pay the taxi for the return trip as well since there probably aren't a lot of people wanting to take a taxi from Le H. to Paris. You can do the math for yourself...
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