The biggest value(production/$ paid) in an NBA market that is capped both for teams and individual players, comes from two main sources:
1. Super stars - because their salaries are capped and usually their contributions are much higher than the salary they are being paid.
2. Rookie contract outplaying their rookie salary. That's why draft picks are extremely valuable. A lot of them outplay their salary by year 3 of the contract. And most rookie contracts are small enough as to not be any type of a detriment to the team even if they bust.
Of course there are others that massively outplay their contract, but those are much more rare because the league usually has had time to evaluate them for at least 4 years(their rookie contract) before they receive their next deal and it usually doesn't miss on great talents past their rookie contracts. Surprises with massive value gained with middling or low non-rookie contracts are rare and are usually triggered by injury concerns(i.e. Curry's contract, but now even if he was at max deal he'd still be a bargain).
This is why teams that have either superstars or great players on rookie contracts massively raise their level. Gaining value in those critical areas allows you to spend the rest of the salary cap in filling in the roster with players that usually would play at about their market value. For example, I would say Hayward and Favors play at about their market value.
Right now we have no super stars and we have 1 player on a rookie contract(Rudy) who is massively outplaying his contract. That's usually not enough.
For example, the Warriors right now have 1 superstar in Curry(arguably 2 if you include Bogut), 3 players in Klay, Green and Barnes that are massively outplaying their rookie contracts. That's why they can afford to pay 10-15 million each to several subs and have an all around great roster. And that's why they won't be able to keep all of their pieces once Green and Klay get max and the time comes for Barnes to get paid.
I hope you don't misunderstand this post to mean - picks are always better than an established player who will most likely play to his market value. I am not saying that. I am saying there are nuances in those considerations and all kinds of stuff factor into them - at what point of the development of the team we are, what is the make up of our current roster, are we at the point where we are set with our group and we are ready to contend, or do we need more young pieces who are likely to outplay their contract, what are reasonable expectations for our other young pieces, or even our non-rookie pieces(are they likely to outplay their contracts in the next several years), etc.
I personally wouldn't mind trading the pick for Barnes, I think he's probably better than what we can get at 12 but you also have to factor in his contract situation - he will have to get paid next year and would we be willing to do it. Keep in mind that this will be the year when everybody will have a ton of money and he will very likely get close to max money which would be about 23-24M. If we are unwilling to match any offer, is it worth trading a lottery pick for 1 year of him? Similar considerations with Batum(we might lose him for nothing in 1 year time)...