Something no one has mentioned yet is that most DO schools (other than the University of North Texas's COM and maybe 2 others) are generally more expensive than MD schools, especially in-state MD schools. Furthermore, I've found that the DO schools to which I applied generally have less institutional support than MD schools.
Tuition is actually comparable. It's not a MD/DO thing, it's a private/public thing. Tuition at private MD schools are just as ridiculous as private DO schools.
Tuition for PCOM (2012-2013) is $42,965
Tuition for LECOM is $29,820 for 2012-2013 (same whether instate or out of state)
Tuition for Jefferson Medical College (2012-2013) is $50,236
Tuition for Temple University SOM (2012-2013) for their in-state residents is $43,654 ($53,468 for out of state)
Tuition for Drexel COM is $47,980 (estimated, 2012-2013 from their website)
Tuition for University of Pennsylvania SOM is $47,090 (2012-2013)
Tuition for Penn State College of Medicine instate resident is $41,860 ($49,984 for out of state residents)
Tuition for University of Pittsburgh instate resident is $44,726 ($45,846 for out of state resident)
As you can see, in Pennsylvania, the DO schools are cheaper than the MD schools.
In NY, NYIT-NYCOM is $48,490. But NYMC is $47,890, Mount Sinai is $41,238, and Einstein/Yeshiva is $42,833. SUNY Downstate is $29,530 for instate residents (2 semesters), but $54,650 for out of state residents (2 semesters).
So as you can see, it's not so much "DO more expensive, MD cheaper" but more private vs public, with DO schools being comparable to MD schools when it comes to tuition.
However, you are right that there are more institutional support from MD schools (many factors, including larger alumni base, larger endowment, etc). DO schools that have been around for decades/centuries have larger alumni base (and more institutional support) than schools that was started within the last decade. So while tuition may be similar, depending on the school, some may offer more institutional assistance than others. UNECOM, Western/COMP, and WVSOM are known for their high student loan debt (2010 graduates had debts of $213k, $203k, and $229k respectively). But they are comparable (in terms of debt) to other MD schools, such as Temple ($201k), Drexel ($205k), GWU ($205k), NYU ($182k)
So basically a pre-medical student, looking at the MD vs DO debate, can't use MD vs DO when it comes to cost of attendance and institutional support. They will have to look at each individual schools that they are interested in, to see both tuition, required fees (which have not been mentioned, just tuition information have been posted), plus cost of living (ie living in Kirksville is cheaper than living in NYC/Long Island), along with possible institutional support (grants/scholarships).