The
$1 app for IOS kind of does this: take a picture, and it will use OCR to read the text out to you. And some Braille Displays can pair with the iPhone by bluetooth. There's also the
$1, that looks like exactly what you're talking about, just with speech not Braille. Personally, I use the KNFB Reader app on IOS, so I'm not likely to carry around a different device just for reading printed text. Plus, the inclusion of Braille means the battery life will probably be not that good, meaning it'll never be charged and ready to go when I need it.
But if you can figure out how to make cheap and tiny Braille displays, I don't think you could go wrong making a Braille display case for the iPhone. IE put the iphone in the case, and then your iPhone has maybe two lines of eight braille cells each, just as though they were built into the iPhone. Connect it directly to the lightning port, meaning no hastles with bluetooth pairing. Plus, when I connect the case to the charger, it will recharge itself and recharge my iPhone, meaning I only need to remember to plug in one device, not two. Yes, it would make the phone a little larger and a little bulkier, but that would be a sacrifice worth making; most people use bluetooth headsets these days anyway, so it isn't like we need to hold the phone up to our faces anymore. And then we have the best of both worlds: an iPhone with whatever apps we want, with a Braille display nearly built-in.