I suppose you could use something like limetext[^] or sublime[^] but still not sure how you would go about editing the .XIB or storyboard files.
If you are concerned about cost then maybe you don't realize that XCode is free. I have tried other routes but I personally like working with XCode.
If you just want an alternatives there is AppCode[^] by the same people who did IntelliJ[^].
And if you are comfortable with using or learning either Swift or Objective-C and would rather work with C# there is of course Xamarin[^] which of course has a cost. I have not looked at lately but it used to require XCode to be installed for developing XIB and storyboard files.