The history of baseball is littered with guys who people thought were going to be great but didn't pan out for one reason or another. Unless you have perfect clairvoyance (if you do, I'm not sure why you're not out there making a ton of profit with it), you can never be 100% certain how a player's career will pan out. It's easy to say that we shouldn't have trusted guys like Turnbow, Henderson, etc. because we have the benefit of hindsight, but at the time I'm sure there were plenty of people back then who at least wouldn't have guessed that things would fall apart so suddenly for them.
That being said, I'm not saying that we should only offer a bag of balls for the guy- the Padres have the right to expect at least a solid return for him. But I think it is unwise to just readily hand someone like Pena for him.