Of course, the initial price of a piece of software is not the only consideration when it comes to 'total cost of ownership'.  With Joomla, the only extra thing that must be paid for is website hosting. Having said that, it is possible to arrange free hosting, but such services usually involve invasive adverts on your site, and even if the service is ad-free, the performance is likely to be very poor - which will reflect badly on you. If you are a UK company, there are distinct advantages to hosting your site in the UK. There are other things to take into consideration though.

For a company of any size, acquiring a new software package represents a certain degree of commitment - there is the initial learning curve, cost of further development if you want to go beyond the core features, perhaps some design work if you want to create a unique look-and-feel rather than use one of the many free templates that are available.

All of these things should be taken into consideration before deciding on a solution.  Open source software such as Joomla almost always proves to have a lower total cost of ownership than commercial software - because you have a huge head-start with the software itself being free.  This also gives you the freedom to try it out without risking anything.