When your major in your undergrad and masters is the same, the uni will definitely weigh your masters grades more. Since your major in your undergrad was in Math, which is definitely harder than economics, it may not count much against you. Try to explain what happened in your undergrad. If your grades were poor in a particular year, then explain what happened. It would be harder to explain if you did poorly throughout your undergrad. Also in your math degree it would help if you did well on modules like real analysis, topology and statistics related modules since these are given importance. If you did badly on pure math modules and did well on the above listed ones, you should be fine. Also try to get a good gre quant score. At least 166. I would encourage you to apply. With good references from econ profs and good gre scores you might have a chance.
All the best!