Note that playoff/relegation game revenue is split between participants, two-thirds to the home team, one third to visiting team.
Teams in either the playoffs or relegation series get the extra game income, but they also have to pay player salaries. Teams in the #5 position (and #8) don't get the added revenue, but they also don't have to pay that last two weeks of salary. Because of this, many managers believe that #5 is a better place to finish, if they feel they don't have a chance to win in the playoffs. The BBs maintain that in the end, the added fan support from making any playoff appearance offset this need to pay salaries, so that #4 is better than #5.
Playoff games do apparently give your players more experience than usual league games, so there's that benefit too (which is of course hard to quantify itself).