It's difficult to tell because there are lots of things to take into consideration. If you're going to hire the taxi, you need to factor in the costs for that as well as whether insurance is going to be included as part of the package.
In some cities, there are more work available than others. So depending on where you plan to work, that might affect how much you're going to take home.
Night drivers or those working during weekends tend to earn more, so if you're going to do unsocial hours, it's going to be better in terms of earning.
Many minicab drivers say it's getting more difficult nowadays to make a decent living unless you're working really long hours, therefore it is important to do your research well before falling for adverts which state that you'll be earning £500 a week as there are fuel costs and other things which need deducting.