I have done this before and it can be done easily in three steps,
Step 1: Export your subsite by using the Export-SPWeb command.
Export-SPWeb -Identity "http://someURL/SomeTeamSite/sandbox/" -Path "C:\Exported Sub Site\SandboxSubsSite.cmp" -IncludeUserSecurity -IncludeVersions all
Step 2: Create you site collection. This is where you will host your new site.
Step 3: Import your Content Migration Package (CMP) file that you exported using Import-SPWeb.
Import-SPWeb "http://someURL/sites/Sandbox" –Path "C:\Exported Sub Site\SandboxSubsSite.cmp"
For detailed step by step guide you can follow this tutorial
Or you can try this free tool available on codeplex,