3 Answers

I'm not going to try to find the real answer.  I'm going to venture guesses.

1.  Perhaps mint was a valuable herb, and to tell someone he is worth as much as a mint was a great compliment.   OR mint plants grew so abundantly they were often considered little more than weeds.  The comment would then be tantamount to saying one was worthless.
2.  The "mint" is where the coin for the United States is forged.  The "mint" would be worth more than I can possibly imagine.  In this case, too, telling one he is worth a mint is to express immense appreciation for someone.
3.  A "breath mint" is small and often given free at restaurants; in this case, being compared to a mint would be discouraging at best.

My personal thought is telling someone he is worth a mint means the person is invaluable, a great asset, one to be treasured. 

Would that be a breath mint????

OBAMA keeps saying that to himself."Kid yourself you just might kid the people.