I'm doing well with Checkbot, a Chrome extension that tests websites for SEO, speed and security problems that has a subscription model:
It's simple in terms of day to day running now that it works well but getting to that stage and the implementation behind it was a ton of work.
I don't think you're going to easily find a project that's easy to implement, requires little maintenance, has low running costs, markets itself, scales etc. You'll have to put in effort somewhere.
I think if you look into the projects being posted in the thread, you'll realise few are simple if you want to make something people will pay for.