By using open source software, we essentially cut down on the hours used to code individual pages and website components by using ready made code etc.
Most of this code, in the form of templates, plug-ins and modules is licensed. Some of these are free but the majority of the licences cost sums ranging from a few pounds upto about £30.
Most importantly, you needn't be concerned that your site will look like someone else's. There are literally thousands of templates, designs, plug-ins etc., and even if we used the same ones for two sites, we can customise them to the point that you wouldn't be able to tell that they had the same origins.