I'll try to explain it(although I'm sure I'll get nowhere).
If you trade 15m worth of players, you do have 15m more to spend. If you trade 15m in cash, your opponent has 15m more to spend. In one trade, everyone is still working with 185m. In the other trade, 30 owners are working with 185m, one is working with 170m and one is working with 200m. If worlds were set up like that from the beginning, no one would take that 170m team.