I am interested in knowing the typical breakdown of an MMORPG's development costs. How much or what percentage of time and money are usually spent in things like engine building, client/server technology development, artwork/graphics, world building, music/sound, game content development, etc? Any opinions or references to other articles/information would be appreciated, thanks.

I haven't seen good breakdowns for specific parts of MMO development, but i have seen "high seven figure, low eight figure" more than once. That puts it in the 8-12 million dollar area. These are for established companies producing AAA titles. Also here's a link for the development costs for City of Heroes.

Forbes Article

Don't forget you need an office and, after work on the game is finished, customer service, legal advisors (for players suing for no reason at all) and the servers cost money too (both to runa and purchase).

Not directly related to your question, but you did ask for any articles that may be of interest, so you may be interested in having a look through the following (and you may even get an indication of costs from Radu's Postmortem articles, IIRC he mentions it):

Eternal Lands Postmortem Articles: Part I | II | III | IV

MMOG Considerations

Interview with Radu Privantu (creator of Eternal Lands)

A Beginners Guide To Creating An MMORPG