The limit is 200GB, not 16GB.
Give the client "unlimited transfer". If the client ever actually reaches 200GB/mo, tell him he needs a bigger server because his website is so popular, and bill him for an upgrade to 768. Make sure you write down "hardware upgrade" rather than "excess bandwidth" on the bill. After all, you'll need an unlimited amount of CPU and RAM to serve an unlimited number of web pages, but the client only purchased unlimited transfer, not unlimited CPU and RAM.
Of course, this works best if the site uses a heavyweight web application such as WordPress, which won't be able to push anywhere near 200GB without a much more powerful server. Bad luck, though, if the site is audio/video heavy. YMMV.
If, however, you're an ethical person, perhaps not...