Next month I am traveling to country X, where I would need Y minutes of calls, Z text messages and most importantly Q megabytes of data. How can I find the cheapest/most reliable SIM card for my needs?
Your best bet is to check the massively useful website called "Prepaid Data SIM Card Wiki". They have an article on pretty much every country in the world and it's kept relatively up to date thanks to their volunteer editors.
After you visit country X and check out their prepaid offers, make sure to go back to the Data SIM Wiki and update the respective article if something is missing or outdated, so that future readers can use your experience as well.