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Antminer is currently one the best profitable miner with Equahash algorithm . When z15 was new it was under $5k. Now used ones sell for maybe $12k. If it is in a time where each one is generating $2k a month, that's a 6 month ROI. In this current market it is probably 2 years or more.
What I am trying to say is only you can determine a good price. Anything over $4k is paying more than they were new. You have to know what your electricity rate is, what your budget is, what the value of those funds are to you, and what your outlook on the future is. There are no guarantees.
In my personal opinion, z15 supply is short and demand is so high that the price reflects the value of what they were earning last year. Being that overall economy is upside down right now it is hard to justify that kind of cost. I personally would be cautious investing in mining hardware where you do not see a clear path to ROI in under a year. Just like stocks, I find it best to invest when sentiment is low, but if you believe in the long term of it. When you can by the thing at a low cost knowing it will eventually be worth more.
