Platform of Choice, Content, Degree of Customization
The platform of choice is probably the factor that has the most influence on the cost of your project. A platform such as Magento or Prestashop will be considerably more expensive due to costly modules, plugins, and the need to implement custom features. In comparison, Joomla, Drupal, WordPress or Opencart may be cheaper due to the many free extensions.
Similarly, costs may vary depending on the amount of content on your website. An online shop with many products will cost more than one with fewer products. A business/personal/corporate website with many pages and a lot of customized content will cost much more than a brochure website.
Speed, SEO, Security
Since proper on-page SEO, performance and security optimizations require considerable time to study, implement and test, costs can increase significantly.
Despite that, the benefits of a fast, secure and well-optimized, search engine friendly website or online shop are obvious and will be reflected in heavier traffic and healthier growth.
Marketing, Branding, PPC, Maintenance
Of course, PPC campaigns, quality branding, as well as marketing and maintenance require a lot of time and commitment. Because of this, costs can be high.