Most history texts only state France had a desire to seek revenge for their defeat in the 7 Years War and that a Treaty of Alliance (1778) was signed. I'm having a hard time convincing myself that France was willing to bankroll a war they didn't need to fight that could have easily gone the other way... all in the name of revenge and liberty of a fledgling nation. What economic incentives did France have that most texts aren't talking about?
First of all, France and the crown of France are two different things. You need to clearly differentiate between the crown, the estates, and the people of France.
The American colonists received three main injections of money and material from France during the war:
(1) A private subsidy from the crown. This was a large gift engineered by the foreign minister of France, le Comte de Vergennes and the king himself, Louis XVI. These monies were issued on the personal authority of the minister of finance and were entirely secret from the public. This gift was undoubtedly undertaken with the aim of confounding and causing grief to the British which was the policy of the House of Bourbon, which included the crown of Spain.
(2) The farmer loan. American agents approached the corporation of French tax farmers and obtained a large loan from them collateralized on the production of tobacco.
(3) The Franklin loans. Benjamin Franklin obtained a series of large loans from Treasury of France as well as a guarantee of several Dutch loans by the crown of France. These loans were public matter, approved by the estates implicitly. The motive for these loans the popularity of the American cause in France.
So, taking a suggestion for the comment above, I will make and answer to this.
The British/English and the French had been going at each other's throat for a long time. Just reading from Wikipedia's list of conflicts between the two nations it is very obvious that they did not like each other. Not long before the Revolutionary War, the French had been defeated by England in the Seven Years war and had lost their colony of Canada to the British. With the Revolutionary War, the French saw a golden opportunity to heap on a victory on top of a British embarrassment. So, the economic incentive of the French was better reputation among other nations and being able to defeat one of, if not her worst enemy.