How much " do" houses cost where you live - why we chose do instead of does ??

Question

How much " do" houses cost where you live -  why we chose do instead of does ??

Answer

"Houses" is plural so you must use "do". If you said, "how much does that house cost?" (singular), you would use "does".

Answer

I do
you do
he/she does
we do
they do

"How much do houses cost where you live?"  - houses = "they"

Another way to ask would be,
"Houses; how much do they cost where you live?"