Following your notation, consider our new price will be $x+y', where y' is the mining price difference.

You are right my claim that y'<y is slightly weak (was based on "gap is not negligible" assumption, see below).

"gap is not negligible" - means if y' gets near to y, then y will get even higher and there will be always a market gap, which I think you disagree with.

Based on your suggestion, on extreme scenario I would soften my claim to y'=<y, without us making profit. :)