Can I publish my game on Google play for free ?

I want to publlish my game to google play for free. Can I do it with Unity ? Also, can I publish my game to IOS, Windows 8 phone, PS, PS4, Blackberry for free? When we try to publish any apk file on google play, it ask for $25. Can I do it free of cost with Unity ?

The cost here is completely separate of Unity, you will face these costs regardless of what tools you use to make your game. The cost that Google is asking for (just like apple will) is to register as a developer with that platform. The Google developer cost is quite small and the Apple Developer Program costs slightly more (I think its something like $99 a year).

These costs are most likely to cover their costs for hosting, distribution, etc of your games. But I’m sure each platform will outline somewhere what those costs are for.