How much does it cost to build an apartment complex for 50,000 people?
Millions, easy. How much exactly will all depend on where you build it, how big or small you build it (big rooms, small rooms), what you build it out of (I.e.construction materials and how much they cost), how close or far it is to sewerage, electricity and water, how expensive or not the construction crews are, and how fast you want it built.
Unless you know all those things, have architectural plans all drawn up (which too are all a part of the cost) and have an idea how much it is going to cost to get consents and permissions to build, you really can't figure out exactly how much. And even then, there could be delays which make it cost more. You never find out the true cost of anything like this until it is completely finished - anything up until then is an estimate or guide. But a complex to house 50,000 - yeah, its not going to be cheap.
In Las Vegas Nevada?