Around $50,000 per year. The Glassdoor website lists salaries as reported by those working as civil engineers. You can look at the salaries per years of experience and per location. "Straight out of college" translates to entry level at Glassdoor. However, my caution would be to avoid choosing a career based upon how much money you think you will make. It has been my experience that those who make the most money love what they do. That is, job performance tends to be affected by how happy you are. No amount of money in the world will make you happy if you hate your job and if you hate your job your performance is likely to suffer. Also, some professions start out with lower salaries but have a higher rate of increase as you gain experience (e.g., start out at $50,000 and then in 10 years make $80,000 or start out at $30,000 and in 10 years make $100,000).
Questions to consider - what exactly is it about math that you enjoy? What professions include these aspects?