Any TPU (thermoplastic polyurethane) case that extends higher than the screen will protect the screen against all reasonable drops, and they're available for $10-$15. (I use an iBlason on mine - it comes with a belt holster.)
I've only dropped a phone once (actually tossed it across a wet blacktop parking lot), but that was a Motorola 7868, and I think it would have taken at least a small tank rolling over it to cause any damage. But I still keep the phone in the case when I don't have to have the back off. Even a 2 foot drop onto my plastic floor mat could probably cause damage. (Probably more than dropping it to concrete if the law of inverse perversity holds up.)