As a monolingual English language speaker who has reached adulthood, you will see very little return on your investment in learning a new language.
Here is the link to the abstract and full text of a paper written by an MIT economist that examined, among other things, expected income gains of learning a foreign language:Links:
Abstract Full Text
Most relevant chart here:
Notable is section (C) - your returns for knowing Spanish in state with significant numbers of Hispanics is negatively correlated with increasing number of Hispanics. Why? Because your lesser Spanish ability is in even lower demand when there are plenty of other Spanish speakers with higher English fluency than your Spanish.
As a native speaker of any language other than English, acquiring English boosts your earnings potential (not to mention prestige) by an enormous amount. The
If you've worked and lived abroad, you already know this at a gut level. Except in very limited circumstances, most employers are not interested in hiring an American who speaks the local language, whether it be Spanish or Russian or Arabic, at a low level of proficiency. And it will be low level proficiency compared to the local guy who speaks English. Unless you spoke that language as a child, you will almost certainly never have high level, business report or academic paper writing skills.
There are plenty of good reasons to learn additional languages, but if you speak English, economic gains aren't one of them.