I was wondering whether someone can explain the household production function (HPF). Specifically, the variant presented in Patanayak et al (2005). The paper uses the HPF to determine household willingness to pay for improved water supplies. The HPF model theorizes that averting expenditures (coping costs) are lower bound to willingness to pay (WTP). However other people (e.g. Orgill-Meyer et al (2018)) talk about how including sunk costs (expensive tank assets, pumps, etc) when calculating coping can reverse this relationship causing coping costs to be upper bound to WTP. However, Patanayak et al, 2005 includes amortized sunk costs in his coping cost calculations and the HPF premise still holds.
This is confusing me a bit.
I know the question is elementary but I am really not an economist. Can someone please clarify this?