Since Windows-NT is not updated frequently, you may wait years for bugs to be fixed. All necessary documentation is included. Support is available for free or for very low cost. There is no user licensing, so you can quickly bring additional computers online. This all adds up to a very low total cost of Ownership.
Several companies offer commercial aggregations at a very low cost.
Applications and Documentation is available for little or no cost. There are no licensing restrictions, so Linux can be installed on as many systems as you like for no additional cost. Linux's total cost of ownership is very low. Even basic applications cost extra. Documentation is expensive, and very little on-line documentation is provided.