A lot. Economics are different in the 23rd Century. Most humans have stopped using money by this point. Without a better understanding of this we'll never know what the cost of the Excelsior class ships is. Either in money or other terms
In the Star Trek: The Original Series episode "Mirror, Mirror": "How can you run a fifty-million-credit starship like it was a pirate vessel?" indicating that either the development cost or the market value of the Enterprise (NCC-1710) was valued at 50,000,000 credits.