Someone I know was talking about 600gb hard drives and his description of the cost was "salty". When I asked him to clarify, he told me it meant that they were expensive. I have searched and can't find any reference to it being used that way. Where does that definition originate? Is it a regionalism?
The opposite of "salty" in this context would be "sweet" (for the buyer), that is cheap.
"Salty" (especially in excess), implies "unpleasant," which (for a buyer) in turn implies "expensive."
Although an old question I feel I might have something to add to the above answers and possibly help other confused visitors.
I don't know if 'salty' is used in the same sense in other languages, but at least here in Finland, we might use it like your friend did: "This apartment is way too small for the rent to be that salty", meaning that the rent of the apartment is too pricy and doesn't correspond to its surface area.
So if your friend was foreign (perhaps Finnish) he would have used a literal translation from Finnish to English in his sentence. It sounds a bit off in English because it's not a known or, at least not commonly, used phrase.
I've never heard this in a modern context, but in that past, salt used to be a very valuable mineral (because of it's abilities to preserve food), so maybe to say something is "salty" is to say it is valuable, like salt, though this might have made more sense hundreds of years ago.
Its origin might be related to the high cost salt had in the past. In Spain "salty" is an expression frequently used (En esa tienda los precios son salados = In that store prices are salty), so it might have been a literal translation of that expression?
The most common slang meaning closely related to OP's example is salty = crude, [slightly] pornographic. The high, excessive meaning intended here is very similar, but I think this usage is not widespread.
I haven't looked anything up, but I imagine that in earlier times before widespread refrigeration, canning, etc., it would be quite common for foodstuffs to be excessively salty. This would quite naturally lead to a strong association between the word salty and the concept of excess.
Possibly fanciful, but it wouldn't surprise me to learn that children's well-known aversion to highly-flavoured foods might have encouraged the salty = smutty sense (not suitable for the young ones).
Given the context of talking about hard drives, it is possible the usage came from the gaming world. Although several sources talk about potential origins of the phrase, personal experience and some other sources make me think that currently this term is mostly used in the gaming world to mean "upset" because they are being beaten, sometimes to the point of toxicity (verbal and textual abuse of other players).
The impression I would have if someone described a hard drive price as "salty" would be that having to pay that price would make them "salty," or upset to the point of pouting about it.