Move sub site to site collection

This can quite easily be performed using PowerShell. It is, however, an absolute requirement for this solution that the 50 subsites are site collections (as Darko suggested, too):

1. Create a content database

New-SPContentDatabase -Name <db_name> –WebApplication <app>

2. Determine disk space used by a specific site collection

$used = (Get-SPSiteAdministration -Identity <sitename>).DiskUsed

3. Move site collection to another database

Move-SPSite -Identity <siteName> -DestinationDatabase <db_name>

Pre-Reqs: Db must be attached to the same web app and physically on the same server

Post actions: iisreset required.

Let me know whether this did the trick.