There are a few differences in how the figures are compared here. With a commercial building like Trump World Tower, the figure is for semi-finished space. That's the cost to erect the main structure, build the lobby and other common areas, shell each office (4 walls and a dropped ceiling), and sell the space. It does not include the cost to fit out an individual office space. If you bought a floor and wanted it done in Marble, that would be on-top of the $300 million cost, paid by the tenant. It's not cost to the investors in Trump World Tower, so isn't in the $300M figure. If somewhere someone tallied all the construction and build out costs for all the tenants of that building, it would substantially higher.
In a single user data center, the costs to build include the shell, power and the fitment of the space. To use some official numbers from a builder in the data center marketplace, CBRE suggests "Data center construction costs average $295 per square foot ($150 to $200 per SF shell, $12M to $18M per MW thereafter depending on the required design resiliency) ".
1.4 million square feet and $300 per, that's a $420M for a shell. I would hope a project of that size could get some economies of scale and come in at least 20% cheaper than that figure, but it really depends on some of the features a tenant might want.
I suspect you could run shell costs from near half that for a "bare bones" setup, to near double for some of the fancier features possible to add (biometrics on every door type bells and whistles).
The big question, is how much power (and cooling, they go together). Low power equipment might require 75 watts/square foot (105 MW), giving a power cost (using the low figure of $12M/MW) of 1.26 Billion; and high power equipment at 300W/square foot (420 MW) would be 5 Billion! Facebook has actually been a pioneer in reducing these costs with it's Open Compute project to make for more efficient setups. This should reduce their power cost well below the average, perhaps shaving 20-30% off that figure as well.
There's one last thing, what about servers? If it's a single tenant data center some folks might include the servers for such a data center. Conservatively 40 servers a rack, 30 square feet per rack, the building could house 1.86 million servers. At $5000/server, that would be another 9.3 billion!
Facebook claims just over a billion active users, or about 537 users/server, if this was their only data center. I'll let the rest of the crowd here debate if that's a reasonable amount of infrastructure per user, or too low, or too high.