Most Expensive Desserts in France?
I am still not sure, if you mean a dessert in a restaurant or a fancy cake you buy at a high end patisserie such as Pierre Hermé, Fauchon, Jean-Paul Hévin, Dalloyau, etc.
You can check Hermé's page for prices of goodies he ships (English version under construction):
If you mean at a high end restaurant, prices for very sophisticated desserts that one cannot even describe, will easily run between 20-30€.
Curious? Just read some menus of the food temples on this web page:
Personally, I not of huge fan of Hermé nor Aoki, but do prefer a lovely lemon or fruit tarte from Gérard Mulot, some macarons from Ladurée or a nice gâteau from any other good neighbourhood patisserie.
The best restaurant desserts aren't as expensive as you'd think - or maybe it's just me. I worked at Alain Ducasse at the Plaza Athenee and prepared the most expensive item on the menu - 120 euros for a caviar, scallop, and caviar dish - while the desserts were only 25 euros each. I saw the amazing work and artistry that went into them and am still shocked at their relatively low price. And yes, they were all delicious.