155 billion in 2010 dollars, and we would be pretty much starting from scratch. Most of that money was spent on procurement and operations. R&D was a small part of the total budget.
Obama's proposed budget for NASA is $18.7 billion for 2012, less than that ($18.0 billion) in 2013 and 2014. About 1/3 of NASA's expenditures go to human space flight, not all of which will go to your back to the future / redo Apollo program. Even if we splash the ISS, kill the JWST, its hard to see more than 6 billion a year going into developing, procuring, and operating a new (old) rocket. We maybe we could redo Apollo in 20 years or so.