The above graph shows how majority of users create websites using WordPress. This is probably because its easy to setup and does not require any coding. Also there is a very wide range of additional functionality you can add to the site using plug-ins. It can be incredibly easy to build a WordPress website once you know the process.
The Top 3 CMS by Market Share
Wordpress is a free open source content management system based on PHP and MySQL. It allows for easy layout changes using Themes and you can easily add complex functionality to your website using plugins. WordPress is used by more than 60 million websites. Now WordPress CMS and WordPress.com are 2 different things entirely. WordPress CMS is a fully customizable content management system whereas WordPress.com is just a web user interface that allows limited functionality to building a website. Also WordPress CMS requires hosting plan and WordPress.com just charges a fee for using their system.
Joomla is the second most used CMS and has been used by approx 58 million websites. Its complexity level is between WordPress being easiest and Drupal being most complex. Joomla web framework is similar to WordPress using PHP and MySQL. Check out the benefits of using Joomla here.
Drupal is the most complex of the 3 top CMS but offers the most powerful. Drupal requires advanced technical knowledge of web coding languages like PHP, HTML and others. The main benefits of Drupal are its open source, customizable, capable of rapid deployment, it is an enterprise class software with extensive API support and is SEO and Mobile friendly.
Cost of Website
Wordpress and Joomla are similar in they are both free to use. Costs will likely come from buying premium themes or plugins, These CMS are simple enough to use that you may not need to hire a webdeveloper to help build the site.
Drupal is free to use but more complex and will likely required budget for hiring a web developer.