What level spell do you have to cast to get permanent deafness? Blindness/Deafness, second or third level, one target.
Thunderstone, 30 GP to get a better effect than a second or third level spell because it has a 10 foot radius and can affect multiple targets.
As you say, getting someone to cast Deafness would cost 60 GP (or 150 GP if you could only find a Cleric to cast it) and then it would only affect one target instead of multiple ones.
Personally, I think making a 30 GP mundane item (that any thieves guild can buy in quantity) more potent than a second or third level spell is a very dumb ruling. JMO.