My computer has several GPUs which is what is used to 'mine' cryptocoins... they are a couple years old so not optimal, but they can mine ~5$ a day worth of Zcash crypto coin... (given the current cost of electricity to run the GPUs at full power and given the current value of ZCASH- 1 year ago 1 Zcash was $40, today it is $432)

If I were to spend $3,000 to replace my GPUs with the latest versions I could mine ~$10 a day and if the value mined stayed the same it would pay for itself in less than a year... BUT the difficulty of the mining calculations grows with time, so the same hardware will mine less and less of a coin per day- so the value of the coin would have to grow at the same rate for the value mined per day to stay the same.

Additionally the crypto coin market fluctuates wildly- so any 'investment' in hardware is probably risky/foolhardy/self-destructive - I am only trying it because I already had the hardware for 3d-rendering...

There are a LOT of people doing it, and the energy used to mine just Bitcoin is huge:
https://www.theguardian.com/technology/2018/jan/17/bitcoin-electricity-usage-huge-climate-cryptocurrency
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itcoin’s electricity usage is enormous. In November, the power consumed by the entire bitcoin network was estimated to be higher than that of the Republic of Ireland. Since then, its demands have only grown. It’s now on pace to use just over 42TWh of electricity in a year, placing it ahead of New Zealand and Hungary and just behind Peru, according to estimates from Digiconomist. That’s commensurate with CO2 emissions of 20 megatonnes – or roughly 1m transatlantic flights.

That fact should be a grave notion to anyone who hopes for the cryptocurrency to grow further in stature and enter widespread usage. But even more alarming is that things could get much, much worse, helping to increase climate change in the process.

Burning huge amounts of electricity isn’t incidental to bitcoin: instead, it’s embedded into the innermost core of the currency, as the operation known as “mining”. In simplified terms, bitcoin mining is a competition to waste the most electricity possible by doing pointless arithmetic quintillions of times a second.