How do I estimate my running costs before buying any hardware?

Estimate the cost side before you commit, because it is the part you can actually predict. Take the hardware's realistic wall power draw, not its spec-sheet TDP, and multiply by the hours you plan to run it to get daily kilowatt-hours. Multiply that by the rate on your real electricity bill, including taxes and delivery charges. Add seasonal cooling if you run in a warm space, plus a small allowance for hardware wear. That gives you a defensible monthly running cost in your own currency. You can do this entirely before spending a cent on mining, which is exactly why it belongs at the start of any honest evaluation rather than after you have already bought in.