This is what newbs fail to calculate. If the ROI interval shortens that means the coin is making good profits and even people with marginal electric rates will now jump in. That happens and the difficulty will go up. So while the fiat return may be the same or perhaps even better, the eth return is typically worse. The only time you can beat ETH returns with a rapidly rising coin is when you have a monopoly on specialized mining equipment. In this case granny is turning on her old PC to mine ETH and difficulty is shooting up.
It's hindsight, but better move would have been to buy ETH with the money spent on the 150 cards. OP will never make as much as he could have bought. NEVER. But if he/she has fun mining then who's to stop them. Just know it would have been better to buy than mine. This is true will all established coins in almost every scenario. Buying also affords you option to quickly exit the market should you decide you've had enough profit or loss - opportunity cost.
Very good point and to add to that, I guess the op is having too much work, rigs crashing, fans need to be replaced, gpus dying, other issues that need to spend a lot of money on and also the fact that it might even burn the house down if no precautions were thought up.
As ETH price rises difficulty rises, I mean, before all this rise eth was around 1721, right now is at 2168 and people are holding out for new amd cards I take. So only the eth he mined prior to the rise will be worth more if he did not sell it. Bullmarket is pointless to mine cause the coin rises more than is worth mining by ten to 50 times, bearmarket is pointless because you could buy the coin far cheaper than mining it, so in the end mining is just a wasteful workload. Now if you have 1 to 5 gpus as a hobby then is fine. You are not going to lose your time and time is the most valuable thing in life.
And there is that thing to resell the crap of it if you dont want to mine anymore, reselling old hardware is hard and not worth these days, sometimes is even better to throw away in the garbage. It saves a lot of time.
https://etherscan.io/chart/difficulty