It depends. The construction cost is a fraction of the cost of land in many areas.... start there, then add $200 to 500$ per sq.ft. Plus 12years worth of inflation for average finishes.

it's going to be way more than you expect because by the time you get out of school and start working, your design expectations will be super high, and your financial wherewithal will be super low

You don't even need to be an architect to design your own home, the money saved in not paying tuition fees would easily pay for it all ;)

I always thought it was free to design my own house.  Was I supposed to be charging my wife for it or something?

That sounds like a good first year architecture school assignment.

The non-helpful, but honest answer is it's as expensive as you want it to be. Do you want a 1,000sf house or a 4,000sf house? Are you okay with stick frame or do you want steel and glass? Do you want custom everything or pick stuff out of a catalog? Non Sequitur's response of $200-$500+/sf is probably the best starting point.

it doesn't cost you anything to design a house. Building it, on the other hand ....

Although you will save money by being your own architect the bulk of the price in building a home are actual construction costs.

Construction costs vary depending on where you live/plan to build. Be wary of anybody throwing out numbers when you haven't even said where it is you're planning on building.

Do some research/get quotes with designers/builders in your area to get a better idea the costs.