Student or Learner
Is the expression " best buy" an idiom? what it means?
A number of "best buy" comparisons have been made...
It is idiomatic. Originally it was just the sort of advice a friend might give: 'The best buy [=the best thing for you to buy] is a Brand X.' The Consumer Association has a magazine called Which?, which uses 'Best buy' as a category (combining price, running costs, service, custmer satisfaction... all those good things). Other magazines now do this, but the CA one was the first in my (limited) experience. (In the late sixties I bought an Adler Tippa portable typewriter on the strength of Which? calling it a 'best buy'.)
In your example, 'best buy' is used as a spin-off from that last sense: in order to make a 'best buy comparison' you use some systematic way of measuring price, customer satisfaction etc.