could the very fact of spending one more year to finish college be a disadvantage of getting into a graduate program?
Many students also do a Master's before applying to enter an Economics PhD program. This is generally viewed as a plus (student is more prepared) and not any different from an additional year in a Bachelor's program (provided that additional year is used properly, see below).
I wasn't really thinking about economics PhD until recently so my GPA is not too impressive and I lack in math courses so I've decided to spend one more year to boost my GPA taking upper level courses and get a math minor.
Boosting your GPA is helpful (low GPAs are a quick and dirty way for those looking at applications to throw them out).
But more important than your GPA and what you should try to get out of your additional year are
- Good letters of recommendation; and
- Good grades in important, relevant, and difficult courses (e.g. getting Bs in Analysis II and Econometrics II is better than getting As in Spanish 101 and Calculus I in your additional year).