I think you're being a bit unfair with that list. I know what you mean but Shevchenko was bought by Abramovich and I'm pretty sure Ferguson did his homework on Berbatov at least as thoroughly as Moyes does, he just thought it was worth the risk. (When you've got money to burn, as they did have at the time, you can afford to take on more risk.)
The higher the fee, the bigger risk and also the more obvious loss.
If Utd sold Berbatov for £15m, they'd have lost £15m, or 50% of their fee.
Say we sold Kroldrup for £2.5m, we'd also have lost 50% but £2.5m compared to £15m doesn't look anywhere near as bad.
When you buy players for small fees there is little chance of getting it completely wrong as the fee doesn't warrant that much attention or risk.
The hard bit is to get it more than right. This is Moyes' best/most obvious attribute. Most of our players have risen to way beyond their original fee.
When you have big money it's hard to get it "more than" right as there is no real market for the price to be inflated to, with the exception of the top 5 players in the world.
The trick there is to simply be right, as if you're badly wrong the fall is a lot more spectacular.