1. Heat cast iron pan for like 15 minutes, starting on low and then bump to med-low and then medium after 5 minutes each.
2. Coat the steak in salt and pepper.
3. Add oil. Preferably something with a high smoke point like peanut oil.
4. Add steak to pan, cooking 3-4 minutes per side.
5. In the last minute, add a bit of butter, tilting the pan and ladling the butter over the steak.
6. Let steak rest for 5 minutes before eating.
7. Enjoy an excellent steak.
There's nothing in the $15 price range that will sear a steak as good as a cast iron pan. I don't have much experience with expensive copper/stainless-steel/aluminum pans, so I can't say that cast iron will get you the best steak ever. But for $15, it will get you a better sear on your meat than anything else you'll find in that price range.