This survey was sent out to the fnalgrad mailing list to solicit the information. 31 people responded to the survey. Of all the monthly amenities added to this due to the higher cost of living in the Chicago area, the most common was free onsite housing to those students whose universities rented houses on the Fermilab site.
Some students receive the benefits of partially or wholly subsidized health care as well.
One must surely be careful with the figures presented here. I have done my best to report only the monthly pre-tax income, but the information was provided in a variety of ways, so in some cases interpolation was necessary. Some issues you might want to consider is that these stipends do come from many states with various costs of living and taxes. In addition, some universities pay their summer students quite well after a fairly low yearly salary, while some seem to pay quite well per month, but only for nine months out of the year with summer unsubsidized.