You judge the cost of a trade when the trade is made.
For instance I didn't like the Norris trade when it was made. The fact that Hader ended up becoming a valuable pitcher shouldn't be a factor in evaluating that trade. Giving up a first round pick, a ML ready player and a lottery ticket was an overpay in my view.
I'd feel the same way about the Machado trade if Yusniel Díaz had panned out.
There are too many unknowns involved in baseball to judge things in a future sense.
The Irvin trade was reasonable from both sides.