Haven't been paying attention to the markets lately. When can expect prices similar to those of 2015?
never
litterally.
not ever.
it's like asking for prices below $1.
they won't happen anymore in the entire future.
In fact, $600 is a huge bargain right now and you should buy them while you still can, because soon it will be at least 4 digits. And once we break $1000 for the second time, you won't likely see sub-$1000 prices ever again either.