- I'm not doing a preliminary marketing research because, in the worst case, me being the only user of the service would be enough, since it's a real problem that I'm facing
- Pricing: I was totally guessing, it would not be an application the user would have to interact a lot with and there will just be a fairly low amount of background jobs, so I don't see the pricing go much higher than that, but then again, me being the only user of the service and using it as a portfolio project would be already a great goal
Are you doing this for income or to scratch an itch?
Charging 0.99 for something is WAY to low to even break even once you factor in tax, operating costs, marketing costs and support.
As I said in the previous comment, it doesn't really matter, since I want to solve the problem primarily for me by minimizing the costs, so my current problem is adopting the best technology, marketing is orthogonal for the time being.
If it's useful enough for you to spend all your time building it, and I'm in the same position as you, it's probably worth $20/month, but it's most definitely worth $5/month.
Not trying to harp on this part of the thread, but it's the overwhelming theme of your responses for a reason.
Also, the biggest expense with a side project that you're trying to turn into something more is your time, spend money on whatever helps you ship the quickest. With that in mind, with respect to your points:
1. Buy a theme off of Themeforest for your site - search for "admin" themes as these will have lots of nice predone pages with charts, user sections, etc.
2. You mentioned Heroku - you could use a User management app like https://addons.heroku.com/auth0 - or just implement whichever one system was the easiest. Again, whatever you can push into actual user hands the fastest.
3. For payments to get started Paypal is fine, but was 99 cents per month an actual price you'd charge? That seems crazy low for anything.
Good luck with your project.