A decent amount. Skubal is great. But looking at SP prices, it's going to cost an arm and a leg and probably another limb. You like what Josiah Hartshorn is doing right now? Assume Detroit does too - type of stuff.
Basically, every team at the deadline who needs an arm will be involved. So you need to take whatever it is you think is fair, and then beat that offer because there will be eight teams who are willing to offer what is fair and you need to beat those eight teams somehow. So for two-three months of one of the three best starters in baseball you're going to pay a price that is going to start at "it hurts" and ends at "it really hurts". And then expect to hear a full discourse on here about how this team isn't one player away from a WS.
I'd pay whatever that price is, probably. I think I'd block off Hartshorn's name, however and be willing to pay whatever else it costs. Prospects are going to fail more than they succeed and Skubal is that good.