The main company is www.franceballoons.com,
but they're also quite expensive (180,
225 or 230 euros per person, depending on which of their web pages you
consult). If you know of a reliable but more affordable company, please let
me know.

The pages were written at different times. Considering that inflation
never goes backwards, 230e is the price.

You're are unlikely to find anything substantially cheaper for about an
hour of a baloon flight. Under the best weather conditions, you will
most likely cover less than 15km (in an unpredictable direction). For a
less romatnic, but more predictable and cheaper adventure, consider a
helicopter flight:

http://www.ligeris.com/usa/ta_heli.html