How much Bitcoin could I earn by directly purchasing $15 per month of BTC; or how much BTC would collect in a month with mining having the same cost.
About 4TH would give you how much BTC currently, for an average month? So a few sums up, you get around 0.00024465 BTC / 9.62 USD. Less than buying directly.
on coinwarz.com, you can calculate that, today, with 4TH/s you can earn ~700 sat /day. but it will turn to ~400sat/day after the halving in april.
you can use exchanges like crypto.com or Bybit that allow you to DCA with little amount. You can set the frequency, the amount, and the duration.
Now, 15bucks are approximately 35000 sats. it is better that 700x30 = 21000 sats with mining.
BUT, you can see things differently. If you run the nano 3 at 65W, on a "solo pool", it will cost you 8.22$/month, and you will earn zero. However, if you find a block (you are a very lucky man) it will be ~4BTC instant.(after halving).
It is a lottery ticket that run every 10 minutes.