This is the question that’s a tough one to nail down since the range of prices is so broad. You can spend very little on a portable barbecue, or you can stretch to a professional level beast with multiple functions and heat sources priced in the thousands.
Most people find somewhere in the middle suits. So spending just over a hundred or a few hundred will generally get you a barbecue fit for most purposes.
Take a look at fuel costs, too, as gas and coal varies. You'll want to factor this in, especially if you’re planning to use the barbecue a lot.