According to various news outlets, the retail price for Windows 10 will cost the same as its predecessor, Windows 8.1 and Windows 8, which is as follow.
Windows 10 Home: $119 (US) | €135 (Europe) | £99.99 (UK)
Windows 10 Pro: $199
Also available is Windows 10 Pro Pack. Windows 10 ProPack allows a system running Windows 10 Home edition to upgrade to Windows 10 Pro edition and enjoy all the features of Windows 10. According to Windows 10 upgrade path, not everybody will get Windows 10 Pro by default, as you only get Windows 10 edition which corresponding with your current Windows 7 or Windows 8 license. So the Windows 10 ProPack will come in handy.
Windows 10 Pro Pack: $99
The price reflects the full version of Windows 10 retail products. The price is slightly more expensive than OEM versions of Windows 10, which mainly targeted to system builders and have some restrictions such as non-transferability as the installed Windows must not be installed or transferred to another PC.
We expect Microsoft to reveal another cheaper pricing for Windows 10 Upgrade versions in future, which applicable to systems that are running Windows Vista, Windows XP or older, for users who missed the free upgrade offer, or for users who decided not to upgrade.
For Windows 8.1, the corresponding price is $119.99, $199.99 and $99.99 for Windows 8.1, Windows 8.1 Pro and Windows 8.1 Pro Pack respectively.