Is research experience in a different field relevant for PhD application?
While field-specific research experience is always the best kind, I think there is a role for good research experience in any related field. Close-by disciplines that share similar research methods and approaches will carry much more weight than something much further removed. (For instance, as a chemical engineer, I wouldn't put much stock in somebody doing sociology or clinical medical research, but somebody who had a background in research in applied mathematics or mechanical engineering would be potentially of interest.)

So, if you're going to go into an area of economics where your mathematical statistics background will be of benefit, I think that will (or at least should) be viewed favorably by an admissions committee.

That said, you will want to make sure that your research supervisor understands that you're applying to economics programs, and tailors her letter of recommendation accordingly. Also, if you have a lot of experience in a different field, you will need to make clear to the admissions committee why it is you want to pursue the new field.