I helped a kid recently with a similar budget put a sub in his car. He already had a decent head unit, but he only had 300$ that his parents would let him spend. Ended up doing a cheap blaupunkt 12 inch sub and blaupunkt amplifier from Walmart, with some 6.5 inch skar speakers. We just ran the speakers off the head unit with no amp. Less than 300$ for sub, amp, speakers and amplifier kit. I built a quick sealed box for his sub for 50$ and he had still money left over. It wasn’t a bad sounding little setup honestly, and the cheapest system I’ve ever helped someone with. It was his first sound system and he was really happy with it. I would’ve put that same equipment in my car if I had a low budget, except maybe the speakers. In hindsight, I would’ve looked for some different front speakers other than skar, but they weren’t horrible for 50$.