It really depends on the size and the quality.
Anywhere from $100,000 to $2,000,000
If you build a cheap building, you'll have problems down the road. Also, if you're a franchisee of a place like McDonald's or Taco Bell, you will likely be required to build to a certain standard which may cost more. Ultimately, customers like to eat somewhere nice. I don't want to eat at a place that clearly was a 7-11 and they just put up a banner and some cheap paint covering up the old stuff