Hispanic in the United States + fluent in Spanish = 2.7% higher average salary

That's according to a study here released in the United States on the economic value of Spanish. A pity it didn't take a look into the value of fluency in Spanish for non-Hispanics though, as Spanish and other languages are learned in quite a different manner when it is not the mother tongue of one's family or relatives.

The study also notes that Brazil is one of the places where Spanish will be seeing strong growth, as also noted in this post here from August - starting next year elementary students in Brazil will begin learning the easiest language in the world for them, resulting in an increase of about 40 million in the number of people that can use the language. The difference between the current situation now is that Spanish is easy to read and understand, but expressing oneself properly in a language is an entirely different matter, much in the same way that a fluent Spanish/Portuguese/Interlingua etc. speaker can do a good job navigating through the Wikipedia in Catalan but couldn't write a single sentence in the language if asked to do so.

An article on that study on the economic value of Spanish can also be read in Portuguese here.

As for how this average +2.7% salary compares to learning other languages in other countries...I haven't seen any studies on this in the private sector (I admittedly haven't been looking either) but for government jobs in Canada the general practice in some sectors is to give employees a one-year training period to become fluent in the second official language, during which the employee will make 80% of the salary that would be made if fluent from the beginning, so for the public sector this amounts to a 25% increase. Also, if a year goes by and the employee is not yet fluent then employment is terminated, so that's another big incentive to study.