One metric for this base-rate is the percentage of women who are currently members of the Society for Neuroeconomics. In November 2017 this was 39%. An estimate based on NIH RePORTER with keywords neuroscience AND decision making gives the slightly lower 37%.
Old estimates (prior to Dec 2017) were based on authorship of contributions to the first (2008) and second (2014) editions of “Neuroeconomics: Decision Making and the Brain” and to a special issue on neuroeconomics in Current Opinion in Behavioral Sciences. That gave 22% (calculated as the percentage of women among all authors contributing to these texts). The higher current rate may reflect a change in the makeup of the field, or the fact that contributions to the books and special issue were curated, or both.