We are talking about reusing for old phones here- does it really need to be power competitive? When we talk about compute per watt we are really talking about c

We are talking about reusing for old phones here- does it really need to be power competitive? When we talk about compute per watt we are really talking about cost. What is the cost of something modern and power competitive compared to reusing a phone that was already purchased, in a setup that mostly idles?