Why Apple watch Nike+ is in same price as normal watch?
The Apple Watch Series 3 and Nike+ Edition are exactly the same hardware, just different bands and some preloaded software (which you can manually install on a plain Apple Watch).  It's the same price because it's essentially the same hardware in the box

From Apple Insider:


  Last year's Nike+ Apple Watch Series 3 wasn't much different from the
  standard version making it a bit tough to choose between the two
  models.


What's different with Series 4....


Not available in Gold or Stainless Steel
Nike exclusive faces that immediately take you to "complications" in the Nike Training App
Nike specific bands (reflective) geared toward training (can't buy separately from Apple's website; off of eBay, well that a different story)