I think a present-value calculation of the cost to buy+run+maintain would be a more accurate/useful metric (i.e. present value of cash flows https://en.wikipedia.org/wiki/Present_value#Net_present_valu...) - although perhaps it is too sophisticated to be done well in practice for a crowd-data-aggregation project like this. If something costs me $100 to buy, and $10 every year to maintain for 20 years, that is not the equivalent of costing me $300 dollars today - because most of those dollars are spent in the future and need to be discounted by inflation and opportunity-cost investment rates. So it might actually only have a net present value cost of $200 in today's dollars. Somehow you have to account for the time-value-of-money factor in order to properly combine the up-front cost with ongoing operation/maintenance costs.