Collider, let me help you think about this more clearly.
If you have some fiat to spend, you can buy a miner, buy BTC or do something else with it.
We are interested in comparing the first two options.
So lets say you have enough fiat to buy a SP30, which for purposes of this exercise costs (say) $5000 or 9 BTC. These numbers are only approximate and don't really matter for this example.
Now, if you buy a SP30 and you mine only 6 BTC, then you have effectively purchased 6 BTC for $5000 (plus hosting costs)
But if you buy BTC with your fiat you have 9 BTC. And you have it all in hand up front and you have some flexibility deciding when and if you want to convert it back to fiat, whereas when you mine BTC you don't have so much flexibility because you only earn BTC a bit at a time, and you are exposed to risk coming from changing network difficulty.
So, which would you rather do ?
Hint #1: the answer depends on how many BTC you think a SP30 will mine.
Hint #2: no body can predict with absolute certainty what future network difficulty will be, but we can all probably agree that it isn't going to get lower any time soon and there is a good chance it will go ballistic over the next 4-6 weeks of diff changes.
You have to mine more BTC with an SP30 (or any other miner) than it costs in BTC if you want to recover the cost of your 'investment'. It doesn't matter whether you calculate this in fiat or BTC.
Obviously, if you don't care whether you recover your costs, then all this doesn't matter.