Bitmain owns a monopoly, however dedicated ASIC hardware is not easy to make, nor is it cheap. Also the miners become slow and out of date every few years, so they need to make a profit.
Only if you have the space, and infrastructure would owning a Bitcoin farm would be a worthwhile investment.
Its not a complete monopoly, they do have (weak) competition but its there: MicroBT, Canaan, Innosilicon, etc.
Late model miners are expensive, indeed 1500 USD for a late 40T+ model, but you can get an old S9 13Tish model for about 145USD. This is the same Asic miner than in Dec 2017 (yeah, when bitcoin peaked its price to nearly 20k) used to cost 3000 USD.
Still the most important factor is the electricity price. Even is someone gifts you the miner, running it can be too expensive, so you have to think of ways of lowering the electricity first and think about hashrate second. For example, a down-clocked S9 producing just 10T is more efficient than the same miner running at 16T, the watts per terahash factor is critical to lower your power bill costs. An S9 started consuming 100W per T produced, but its now possible to make it consume just 80W per T. Of course a late model miner is already consuming under 60W per T, but those are more expensive.
A farm is a complete separate beast, and i think the lower the profitability the smaller they will become, especially when it becomes unprofitable to mine in any country (including mine).
