Actually, two possibilities, but first note we really wouldn’t use “cars” at all. So . . .
“Which are more expensive, BMWs or Toyotas?”
Or it could work in the singular just as well:
“Which is more expensive, a BMW or a Toyota?”
It’s understood that we’re talking about cars. If you wanted to specify to a (highly uninformed person) that BMWs and Toyotas are cars, you might say: “Which cars are more expensive, BMWs or Toyotas?”
Note one other detail that many native writers make a mistake with: with the plural, there is no apostrophe after “BMW” or “Toyota.” This is a very common mistake, almost to the point that the usage may change the rules someday. But currently, with standard written English, you’d correctly only use the apostrophe to signal possession. E.g., “A BMW’s gas mileage is not as good as a Toyota’s.”