These questions aren't simple to answer.
In baseball, we have very developed cumulative statistics that allow us to compare value. We are able to identify exactly how much of a player's performance contributes to his team's success. The unit of measurement is called WAR, or Wins Above Replacement. The more players a team has that contribute positive WAR, the more wins the team has during the season -- easy. So, when a rebuilding team trades its star 1st baseman for multiple prospects, we compare the "Wins" accumulated by the MLB player over the coming years to what the prospects eventually yield (1st Baseman earns 12 WAR over the next three seasons, while the two prospects combine for 11 WAR over a similar time span -- therefore one might argue the team that traded for the 1st Baseman narrowly "wins" the swap).