Before the bridge the only way to get from HK to Macau/Zhuhai by land was 4+ hours. Now it's 20 mins. There other bridges being built further up the delta (like the Shenzhen-ZhongShan bridge which will cut a 2 hour drive to 10 mins).
You mentioned the cost-benefit. Making is easier for a region of 60 million people to get around obviously has a very high benefit.
Well, think most people were taking the ferry between the two cities (~1 hour ride). In addition, it appears there's a low daily cap in terms of the number of cars from HK that can enter Macau? Also has Macau developed into something other than a gambling destination? I wasn't initially doubting the cost-benefit (though at first blush I am starting to wonder), but the engineering challenges of this incredible project.
You mentioned the cost-benefit. Making is easier for a region of 60 million people to get around obviously has a very high benefit.