It's not going to be cheap. But I believe the Sox have a lot of what KC wants. Talent that is 1-3 years away from contributing at the MLB level. That is the time frame they are looking at for much of their own top prospects to be in KC.
That considered the deal IMO would be something like this,
Kelly has to be part of the package, we might as well get over that now. After that it get's a little tricky. KC's top 20 has a couple 1B at the top. The 3B of the future. A lot of RHP. And a few OF's towards the back end. Their only catching prospect in the top 20 is at #17 and isn't considered to be anywhere close at this point.
So we can take out 1B and 3B prospects. With the amount of pitching KC has the Sox might be able to get away with just one more higher upside pitcher in the deal. So I'll concentrate elsewhere for now. I think a C prospect could have a lot of weight in the deal. Now the Sox have talent at C. The problem for us and the Sox was that it wasn't ready to take over now. KC won't have that problem. So really I think you give them their choice of Catching prospect. Moving on. I'm sure they are going to want a top OF talent. This probably means Kalish, Reddick, Fuentes and Lin. If it's Kalish it's probably only him. they might be able to get 2 out of 3 for the other group.
Kelly+
(Bowden, Pimentel, Doubront, )+
Kalish(or 2/3 of Reddick, Lavernway, Lin, Fuentes)
Catcher+
(the unknown low level guy that always ends up in a deal like this)
I think that is a solid starting point. It has risk and is a steep price yes, but the amount of interest will more then likely drive the price that high.