the math subject test is rather difficult (as are most gre subject tests) and many mathematics phd applicants do not score extremely well on it. i would highly advise against doing so. unlike the general gre, the math subject test covers material one would learn in while pursuing a BA in mathematics, including topics that are not at all pertinent to economists (abstract algebra, number theory, geometry, complex analysis, etc.).
in addition, the signaling value is not particularly high, even for those without a super strong math background. quite honestly, even if you did score well on the test, it wouldn't suddenly make up for a weak math background, especially if you happen to lack the 'necessary' courses.