Very good question.
I suppose anything below $8500/BTC is a good price.
Between $5000 and $8500 is even better and if you can grab it below $5000 it would be a great opportunity.
My personal opinion is that Bitcoin is here to stay, at least for the next few years. The risk being that a technological superior token might render Bitcoin obsolete. Although there are already many cryptos with a lot more advanced capabilities, and so far, Bitcoin is still around strong as ever.
During the 2018 crypto crash, Bitcoin went below $3500 (I can not recall the exact number) and there was a concern that BTC would disappear as it was no longer worth mining. Despite this concern, the majority of miners kept working at it. I guess this tells us that there is not really a minimum price beyond which Bitcoin is no longer valuable, but rather if you can get it for an even lower price, all the better for you…It should raise afterwards, at some point. At least, it has always recovered, so far…