Most of the fantasy/Rotisserie guys that follow me on Twitter don't play in keeper leagues. So most of the questions I get surround how much a player is worth right now and have nothing to do with how to evaluate a player's value going forward.
Keeper leagues are quite different. While the ultimate goal is still winning, you don't want to give away a young stud if the end result is going to be a third place finish. While you shouldn't be afraid to trade a young guy away, you shouldn't simply trade him straight up in a needs trade. You're not only giving away the player's stats this year (which should be meaningless to the team playing for this year anyway) but you're giving your opponent a leg up for next year (and putting yourself at a disadvantage).
While you should get more than face value for a dump chip, what the price should be for a dump chip isn't determined by what I have to say about it but entirely by your league's market value. In some leagues, getting a player that produces as much in the way of stats as the guy you're giving up plus a moderate upgrade elsewhere is fair. In other leagues, you wouldn't trade a future chip unless you got two everyday players that were both at least as good as the dump chip who were replacing a dead spot on your roster.
Every league is different. In my experience in keeper leagues, here is what I have found.
1) You might take less back in May than you would in July.
Getting two solid players for a future stud in May versus getting them in July provides more value to your team. Acquiring players for a 55-60 game stretch is helpful but not as helpful as getting guys for 100-110 games.
2) Some owners value young players more than others.
Some owners value young players too much, while others value them too little. Holding onto a young player when you can make an obvious gain this year doesn't make sense. But trading a young player simply because you're desperate to get better and getting too little back doesn't make sense either.
3) Farm guys can be valuable
This also depends on your league's philosophy, but some owners love minor leaguers. Mike Trout and Bryce Harper have probably increased the love of minor leaguers in your league this year, since everyone is simply assuming that these guys are the new normal and not a generational aberration.
4) Pitching is undervalued
Owners tend not to value young pitching until that pitching has become Major League established. By the time this happens, these pitchers are usually past the point where they are cheap keepers and instead become market value players at auction.
As I said above, this is a tough subject to write about because every league is different. Common sense should be your guide. If you trade Eric Hosmer for Miguel Cabrera and Michael Young and then your opponent turns around and trades Jason Kipnis for Ian Kinsler, Robinson Cano, and Justin Verlander, in the context of your league you're probably not asking for enough.