How to model product popularity by their prices and sales volume?
We know that the expensive one is actually more popular then the cheap one


What makes you think that? Apparently you have a reason for that, so start from there, find those attributes - is it high customer satisfaction? positive reviews? repurchase? low retention? 

Gather those attributes together, evaluate them on all products to see what are the popular ones. It'd help to have a proper definition of popularity.

Sales volume can help too, give it a weight to balance the price difference.

Once you have all this in one table, you can start playing with prediction, clustering, trends etc.