Drupal 8 migration will cost you little higher than the money you spent building your Drupal 7 site. But maintaining a Drupal 7 site after the official support has been withdrawn, will cost you much more than migration to Drupal 8.
Drupal 7 will be officially supported till November, 2020. After that, there will be no official security releases and updates. You will have to rely on your vender to identify and fix the issues to keep your Drupal 7 website running and profitable. This may require constant monitoring of website, observing the users and their behaviour, employing various techniques to identify and guard against brute force or similar attacks to exploit any security vulnerability. This costs both energy and money.
Cost of maintaining & enhancing Drupal 7 website
If we take an average rate of US $30 per hour, you may end up spending around US $2000 per month if you have lots of enhancements and maintenance going on. Now, consider if you have to monitor your Drupal 7 website continuously and look for security issues in core as well as contributed modules and themes available on https://drupal.org.
If not monthly, you may spend US $2000 every few months to fix security vulnerability and enhancements.
Cost of migrating to Drupal 8
Consider you want to migrate to Drupal 8 and there will be no cost at all when you migrate to Drupal 9, it will be like a minor Drupal version update!
With same rates – Us $30 per hours, with lots (a lot!) of content, users, other entities, your custom Drupal 7 theme and modules, this can take up to 4 – 6 months to migrate and launch your Drupal 8 website. The timeline is entirely based on the customization you did to your Drupal 7 website.
40 hours per week, 4 weeks per months, 4 months to migrate: 40 X 4 X 4 X 30 = US $19,200
This is an investment that will not recur in years to come and will help you earn more revenue by saving as much as US $24000 per year as you will not have to employ continues efforts to monitor your site for security vulnerability and enhancements required by those issues.
Cost of maintenance & enhancements
At the rate of US $30 per hour, for 67 hours, it will be again around US $2000 exclusively for new features or enhancing existing custom features. But not a single penny for fixing security vulnerability and its effects! That is because soon, Drupal 8 will have one-click minor version upgrade feature which will be available in Drupal 9 as well.
Besides, you will not have to spend to upgrade to Drupal 9. All you have to do is to keep your Drupal 8 website updated to Drupal 8.9 as and when it is released. You are already in Drupal 9!
With one time investment, you can keep your cost minimum at a later stage and generate more revenue and get a stable Drupal 8 website with little or no efforts. It is the time to act now!