This disturbs me. In spite being an avid gardener, I really can't see how watering a couple of plants actually costs any money (or how removing them saves any). With that being said, I too would like to see these studies
EDIT: Here's a presentation through the Green Building Council of Australia that highlights the benefits of indoor environments on productivity. Note however, it is sponsored by a company that offers the previously mentioned contracts.
(PDF) http://www.gbca.org.au/uploads/70/2886/Meet%20the%20Stars-%2...
Here is a study published in the American Society for Horticultural Sciences' HortScience journal: http://hortsci.ashspublications.org/cgi/content/full/43/1/18...