To give you a bit of context, I started Pixelixe.com as a side project and launched it at the end of March this year (2019). Pixelixe basically offers a free or paid (monthly fee $4.99 = Saas model ) graphic editor tool. I built it to be one of the easiest way online to create graphics for social media and marketing purposes (but nothing new there, I am fully aware I have a lot of competitors but there is enough place for everybody). I work on this project during my free time (weekends, evening, holidays)
I spent literally Zero dollars on marketing. Even better, the overall project cost is inferior than $15. Also, keep in mind I am a solo preneur, and I have to do everything by myself (design, bug fixes, business model iteration, communication, SEO, and so on)
If you would like to know more about how I decided to start my own business as a side project and how I choose a startup idea and built it, I strongly encourage you to read my previous blog post where I explain : How a facebook rejection motivated me to start my own project.