Mining for new BitCoin (not trading in existing BitCoin) is expensive in every way. Yes, you can put up a simple 500 USD system and start mining with it, but don't expect to find new BitCoin any time soon. BitCoin is notoriously inefficient to "mine" and this is by design. More or less all the "low hanging fruit" has already been picked.
A system that gives you a reasonable shot at gaining new BitCoin is going to cost you. You still need many GPUs, preferably a motherboard with enough PCI lanes to properly support these GPUs, a processor that can support those lanes as well, a power supply that can support this setup, a case that allows for all this equipment to be mounted in and you will be needing an airconditioned room to put this contraption in. Several thousands of USD such a system will cost you. Still nothing compared with actual mining equipment.
Depending on your geographical location and your power source, you will keep spending a varying amount for the monthly electricity bill. Here in Paraguay, which has hydro source of electricity (and a shitty grid in most of the country), such a system would easily cost you around 300 USD a month (with the current exchange rate). Electricity is quite cheap here.
Even with a rig like that, getting new BitCoin will take quite some time. And with the wildly varying value of BitCoin, it could either be very profitable or a big loss leader for you. Might as well buy a lottery ticket. A lot less effort on your end, admittedly a lot less chance of getting lucky, but the payout can be massive. The environment will appreciate the lottery route more.