Two decades of distance education research

Zawacki-Richter, Baecker and Vogt recently published a review of distance education research from 2000 to 2008 in IRRODL.  Comparing their findings with Berge and Mrozowski's review of 1990-1999 research (2001) struck me that not much has changed in the state of research in distance education.  

What worries me more is that the least studied area is cost and benefit.  Are distance education institutions going the way of correspondence schools as presented by David Noble in Digital Diploma Mills, Rehearsal for the Revolution?   How can we refute or support his arguments if there is not enough research?

References:

Berge, Z.L., & Mrozowski, S. (2001). Review of research in distance education, 1990-99. The American Journal of Distance Education 15(3), pp. 5-9.  Full text available in Practitioner Research and Evaluation Skills Training (PREST) A1 module reading resources, http://www.col.org/SiteCollectionDocuments/A1%20resources.pdf.

Noble, D.F. (1999). Digital diploma mills, part IV. Rehearsal for the Revolution. Retrieved December 31, 2009, from http://communication.ucsd.edu/dl/ddm4.html.

Zawacki-Richter, O., Baecker, E., &; Vogt, S. (2009). Review of distance education research (2000 to 2008): Analysis of research areas, methods, and authorship patterns. The International Review Of Research In Open And Distance Learning, 10(6). Retrieved December 31, 2009, from http://www.irrodl.org/index.php/irrodl/article/view/741/1433