I recently traded Scott Olsen and Joel Hanrahan for Clemens (albeit, Olsen has shown he has some major league ability already).
Frequently being on the side of prospect-for-MLB player trades that one would hope to not be on, I would say that it takes a very good minor leaguer to net one of my major leaguers, even if they aren't one of the best. It doesn't matter to me if a player is the next Griffey or A-Rod, prospects are always several notches below their theoretical counterparts in the majors, simply because they have yet to prove anything in the majors.
For me to trade away a top player for a prospect, no matter how good, there would have to be a good, young, cheap keeper in the mix as well. In fact, given the choice between one of those good, young, cheap keepers and a prospect toiling in the minors, I'll take the MLBer everytime. I always remember that I have something they want, and that I'm giving up talent that's here now for talent that might be there later. I make damn sure I squeeze every last drop out. If someone doesn't give me the package I'm looking for, you can be sure there'll be another contender that will.
For instance, in the Manny for Delmon trade, Young could very well turn out to be a very good player. However, while he might be in the very upper echelon of hitters in the minors right now, it's a lot more crowded in the majors. There is a difference between very good hitter and potential HOF hitter. This isn't saying that Young won't be a very good hitter, nor is this saying that having a very good hitter is a bad thing. Look at it this way: at best, Young will be a HOF-type hitter. At worst, he may fail in the majors. It's a lot more likely that he falls somewhere in between. Right now, Manny is a potential HOF hitter. There is no worst case scenario. You know what you have.
There are two reasons for trading for prospects. The first is salary purposes. The second is upside. In this case, where I assume there are no salaries, it could only be for upside. Except that Young's upside is where Manny is right now. You're basically trading certainty for uncertainty, and that makes little sense.
The solution is to ask for more. I would ask for at least two more good prospects or at least one more very lucrative young keeper. He may not give you all you want, but you should at least drive a hard bargain. After all, Young has no value to him right now. The only reason Young has value is because you're looking toward the future. The party that holds the major leaguer always has the upper hand (except in the case of an extremely large contract). Use that upper hand to get as much as you can.
P.S. I project Young as a 35 HR, .312 AVG guy, based on his numbers thus far.