Keep in mind (you probably know this) that Obama canceled this rocket before.And nothing has substantiatively changed between that rocket (Ares V) and this one, other than the clock being reset back to 2005; when Ares V was ET diameter and used SSMEs.
And the only reason that the SSMEs are back in there is because Congress pushed for them.I believe the law called for STS elements to be used where practicable. That was a huge escape clause which went unused.
Here are some cost estimates for the SLS program:
Space Launch System.
During the joint Senate-NASA presentation in September 2011, it was
stated that the SLS program has a projected development cost of $18
billion through 2017, with $10B for the SLS rocket, $6B for the Orion
Multi-Purpose Crew Vehicle and $2B for upgrades to the launch pad and
other facilities at Kennedy Space Center. An unofficial NASA
document estimates the cost of the program through 2025 will total at
least $41B for four 70 metric ton launches (1 unmanned in 2017, 3
manned starting in 2021). The 130 metric ton version should not be
ready earlier than 2030." http://en.wikipedia.org/wiki/Space_Launch_System#Program_costs
So just for the development costs alone for the interim 70 mT
launcher scheduled to only make 4 launches, that's $4.5 billion per
launch. For 70,000 kg payload that's $64,000 per kg, and that's not
even including the production and operations costs.
If that larger $41 billion number is valid for the total costs that's
$146,000 per kg. A common saying going around nowadays is "the
definition of insanity is doing the same thing over and over and
expecting a different result."
Building large launchers is supposed
to result in reduced costs not
Falcon Heavy Booster: Why Is It Important?
by John K. Strickland, Jr.
"What amazes people is that SpaceX has broken the long-sought 1,000
dollars a pound to orbit price barrier with a rocket which is still
expendable. 'How can he (SpaceX CEO Elon Musk) possibly do this?' they
ask. The Chinese have said flatly that there is no way they can
compete with such a low price. It is important to remember that this
was not done in a single step. The Falcon 9 already has a large price
advantage over other boosters, even though it does not have the
payload capacity of some of the largest ones. The 'Heavy' will even
this score and then some. At last count, SpaceX had a launch manifest
of over 40 payloads, far exceeding any current government contracts,
with more being added every month. These are divided between the
Falcon 9 and the Falcon Heavy." http://www.nss.org/articles/falconheavy.html
The most important accomplishment of SpaceX may turn out to be
that they showed in stark terms that privately financed spacecraft, both
launchers and crew capsules, can be developed for 1/10th the cost of
government financed ones.