Ballon Monte (manned balloon) covers are not especially rare since there was a very large number of balloons that were launched - 66 - and the estimate is that a total of about 2.5 million letters and cards were carried on them. Still, they are desirable and command good prices, especially certain flights that are less commonly found than most.
You can buy them on
ebay every week for between $75 and $175 for most and as much as $300 for the really rare and desirable ones. Ones that contain their original messages tend to be of greater value, and the ones with original contents with really interesting content are the ones that tend to go for the most. Back stamps to prove they were flown are essential. There are reference books on these and if you would like to collect them you will need some of the reference books and the catalog of them, along with the ability to read French handwriting.
Since they command good prices, there are fakes out there, some of which are quite well done so if you are going to be buying them you should get them expertized.
For those who do not know about these, the story is that in the Franco-Prussian war the Prussians had surrounded Paris and laid siege starting in 1870. During the two years of the siege, 1870 - 1871, Parisians built manned and unmanned balloons to carry mail out of the city and over the Prussian army forces and when they landed the mail in them was taken to the nearest post office and put in the mail stream. Of course there was little ability to direct where the balloons would land so they are seen with back stamps all over the place. There were also pigeon mail flights to carry messages on carrier pigeons. These were carried as microfilms in tiny tubes attached to the pigeon's feet. The best thing about the pigeon mail was that it was two-way, both out of Paris and into Paris. The balloon mail was one-way - only out of Paris. Balloons could not be steered so there was no way to get them and mail they carried back into the city.