I saw this question on a forum and it made me pause for a second to think about it. The quick answer is it varies. The sarcastic answer is it costs as much as you spend on it, or how about, it cost as much as you didn’t spend on creating a maintainable automation project.
I have only been involved in 2 other test automation projects prior to my current position. In both I also had feature development responsibility. On one of the projects, comparing against time developing features, I spent about 10-15% of my time maintaining tests and about 25% writing them. So, that is about 30-40% of my total test time on maintenance. Based on my knowledge today, some of my past tests weren’t that good so maybe the numbers should have been higher or lower. On the other project, test maintenance was closer to 50% and that was because of poor tool choice. I can state the numbers because I tracked my time spent. I could not use these as benchmarks to estimate maintenance cost on my current project or any other unless the context was very similar and I can easily draw the comparison.
I have seen where someone might say “it’s typically between this and that percentage of development cost,” or something similar. Trying to quantify maintenance costs is hard, very hard and it depends on the context. You can try to estimate based on someone else’s guess of a rough percentage and hope it pans out, but in the end it is dependent on execution and environment. An application that changes often vs. one that rarely changes, poorly written automated tests, bad choice of automation framework, skill of the automated tester…there is a lot that can change cost from project to project. I am curious if someone has a formula to calculate an estimate across all projects, but having an insane focus on the maintainability of your automated test suites can significantly reduce costs in the long run. So a better focus, IMHO, is on getting the best test architecture, tools, framework, people and make maintainability a high priority goal. Also properly tracking maintenance in the project management or bug tracking system can provide a more valuable measure of cost across the life of a project. If you properly track maintenance cost (time), you get a benchmark that is customized for your context. Trying to calculate cost up front with nothing to base the calculations on but a wild uneducated guess can lead to a false sense of security.
So, if you are trying to plan a new automation project and you ask me about cost the answer is, “The cost of having automated tests…priceless. The cost of maintaining automated tests…I have no idea.”