It's really difficult to say. Not only was the project completed in a series of different parts, but prisoners, soldiers, and foreign workers were also recruited to help with the construction effort - skewing the actual cost.
"Initial estimates put the cost of building the railway at $175 million. By 1904, official accounts said that $250 had been spent. The number was actually significantly more, especially when consideration is taken for the East Chinese line, built across Manchuria."
While the real figure is undoubtedly greater than $250 million, it's also worth remembering that $250 million in 1904 would be equal to anything from $5,080,000,000.00 to $157,000,000,000.00 in today's money!