Software-related Costs
Site Software
TYPO3 and its extensions are open source software. They are provided free of charge.
Compared to a commercial CMS, you save many thousands of pounds before you begin; or alternatively, you have several thousands of pounds more money to devote to the site itself.
This saving would be nonsense if the product did not work, of course; but actually, TYPO3 has a very high degree of functionality and integrity, a clear development programme, and active security team, and extensive infrastructure for support, bug tracking and elimination.
Software costs for the site itself are therefore limited to development and configuration costs; and in many cases you will not have development costs, since there are already several thousand extensions available which may fulfill all your requirements.
Additional Software
In principle your site can be managed, and the content can be generated from any internet-connected computer. In reality, however, you are likely to need at some additional software, particularly if you want to incorporate graphics and other media.