No impact in general. Most grad econ programs don't admit by field, nor do they have quotas by field. Even if they do, it might not be the case that anyone on the admissions committee is in your field. Even if someone in your admissions committee happens to be in your field, it might not be the case that (s)he recognizes your letter-writer or hold him/her in high esteem.
There are exceptions - in my application season, the above actually happened - but I suspect it is extremely rare and it's best to maximize letter quality.