You're right. I made a fairly quick judgement based on the 2 JD/MBAs that I know - both are going into investment banking. I suppose it would definitely come in pretty handy for the law route as well.
As far as the finances, I looked at Northwestern's JD-MBA website:
The estimated total costs for the 2008-2009 academic year (12 months), including tuition, books and materials, and computer equipment is $95,000; tuition alone is $63,000.
I take it that the $95,000 includes living expenses as well. Considering 5% annual tuition increases, that would be a total cost of approximately $300,000 (more than 2x the cost of a Kellogg MBA). Depending on what your savings and internship situations are - I would expect that the average student's debt load would be between $200-250,000.
Also, keep in mind that Northwestern is one of the few 3 year programs out there. Most JD/MBA programs are still a 4 year program. I have no idea about the cost of the others, but I would presume that it's even higher?