I would not trade Garza. He is a top tier starting pitcher in his prime, and we have the money to extend him if we really want to. There's no reason to trade him, IMO. When you trade a guy like him, it's incredibly difficult to get the same quality back, and you're taking a big risk.
One thing that people forget is that top tier talent is way, way harder to collect than mid-tier talent (like the difference between a #1 starter vs a #3 or a #4 starter). It's rare that top-tier talent is available at all. When it is available, you usually have to overpay for it either in $$$ or in years (signing guys way into their mid- or late-thirties). Giving up a #3 or a #4 for prospects is a lot less risky, because the chances are much greater that one of the prospects coming in will one day perform at that level.
When it comes to prospects, even the BEST pitching prospects are a 50-50 proposition. There have been a million "can't miss" pitching prospects who have missed. Even getting two elite prospects in exchange for Garza would be very risky, and no team would part with two elite prospects for him.
The chances that a Jacob Turner will become as good as Garza is maybe about 10-15%, if that. The chances that he will be BETTER than Garza are very, very low (virtually nil). So, to make the trade to work, you need Turner to equal Garza and another prospect to develop into something good, as well. The odds are against it working.
It might be worth trading for Garza if we couldn't afford to extend him, but we are a big market team and we are under budget. That is one of the few advantages we have right now. I would prefer to leverage that opportunity by extending Garza. If we have to overpay a little bit, so be it. We need to collect as many guys like Garza as possible.