You're close, but remember the definition: The CPI or Consumer Price Index is a measure of the
cost of goods purchased by average U.S. household. It is calculated by the U.S. government's Bureau of Labor Statistics.
If 75% is the current homeownership rate, 75% of the country is not purchasing a house. My house could double in price tomorrow and this would not cost me anything. My mortgage payment is unchanged. If gasoline doubled in price that would impact me because I'll buy gas this week, I won't buy a new house for 10 or 15 years.
Fine, just scale the numbers to accurately reflect the relative numbers of home buyers to renters to owners paying a mortgage, to outright owners. Add into the mix the cost of housing, the cost of renting, the cost of mortgages, and of course the cost of property taxes (which apply to all but the second category, although it is folded into their rent). And scale by the percentages, and you have an accurate picture of the "average cost of housing" across the entire population. Certainly a lot more accurate than considering renters only. (Ultimately what is sought is to collect the entire US expenditure on housing, divided by the total population, for the exact average cost spent on housing.) That is
certainly a lot more accurate than assessing rental costs only.
My mortgage payment is unchanged.
Don't forget that there are an awful lot of ARMs out there - their mortgage costs will change, and that is a change in the cost of housing.