Realistically, the bill would have been in the hundreds of millions of dollars and simply adjusting for inflation wouldn't quite get you there either. The reason for this is because much of the craftsmanship which went into the appointments was either by hand or by manual tools used by expert craftsmen.
Such arts aren't totally lost but they are extremely hard to come by and the people who can do it are well aware of the value and the scarcity of their skills.
If a ship with similar appointments to the Titanic were to be built, I would be greatly surprised if the nominal customer could even get a handshake for less then $500million.
That's a good point, Michael, one I hadn't really thought of. I guess you'd also consider the added expense of adhering to modern fire safety regulations, and so on as well. I did a little reading (wikipedia, not the most reliable source, I'm aware) on modern cruise ships, and the Queen Mary 2, for example, cost about $900 million to build. I think your estimate of $500 million for Titanic might even be a bit of a lowball!
As a playwright, I stage my play 'Titanic - William & Mary's Story' way back in 2004. It was a total sell-out. I staged it again in 2009 in a 300 seater theatre. Again, a total sell out. I guess the interest will never die....