The online calculator I found only goes back to the middle of 1901, so 12,000 FF back then is worth about 47,110 Euros today. Going back earlier than 1901 would require some interesting historic research. But a guess would be that it would have been serious money.
If it helps at all, enough to build a canning factory according to Wikipedia.
> Unfortunately for Appert, the factory which he had built with his prize money was razed in 1814 by Allied soldiers when they entered France.