Most of the great premium WordPress themes cost around $70-100. Some companies offer subscription based service where you need pay annually. For example Elegant Themes.
Your doubt is why is it so cheap. You are not the only one they sell these products. Already hundreds or thousands of people might have purchased the same theme. That is how they make profit.
It is not impossible to develop a complete website by yourself but it needs lots of time, energy, experience and skills.
You can ask your client if he is ok with using particular theme. Remember, you have the option to customize the theme if you want. (You should use a child theme for this, otherwise you may run into trouble when there is WordPress, theme or plugin update.)