Of course it depends on the scope of the website project, circumstances around that and personal preferences if someone is willing to pay for a Bootstrap theme or not. In the world of Bootstrap themes there are plenty of resources available to get free themes, templates and other tools.
Like in general regarding website themes, comparing free themes to themes you have to pay for will most often bring many differences to light, e.g. number of styled components, level of detail and other additional styling options.
When getting to a decision if free or paid theme it always boils down to what project you are working on and how much budget is assigned. If it is a serious project you should always aim to get a good paid theme, especially as you can get many popular Bootstrap themes for around 30 to 60 dollars. If it is a project which is not that serious it is always a good option to begin with a free theme. Also if you have knowledge in HTML and CSS to develop your site it might be perfectly fine for you to just use a free theme as you can futher customize it by yourself.
You can get many popular Bootstrap themes for around 30 to 60 dollar
