I don't understand while the 7-th levels doesn't join extra-league tornaments.
Simple sample:
Lawn-mower-Trophy
Semifinal
Prize-money for the next stage: 3000
Glorypoints: 120
Finals
Prize for winning: 4000 (Total 7000)
Glorypoints: 160 (Total 280)

and it's for 4 game.

Mixed: European Professional's Cup

Prize for winning: 1800 (Total 5300)
Glorypoints: 72 (Total 212)

8 games and prize is much smaller, than Lawn-mower-trophy. And you have a small probability that you would lose to 3-4 levels and gain -7 morale. And actually you don't receive good experience for your players, because your team strength on the field is much higher than your opponents.

VIP Tournaments: VIP International Pilot's Trophy
Prize for winning: 2000 (Total 3500)
Glorypoints: 240 (Total 420)
5 games, clear prize is 3000 and slave is only +140 as in lawn-mower.


So if sum up all that i write here:
1. In extra league you earn much more money (i think everybody wants to create their own players etc, isn't it?).
2. The slave is much bigger than in mixed cups and a little bit smaller than in vip tournaments except of VIP International Dice Cup.
3. It's very fast 4 games in such cups usually plays for 15-20 minutes, for example Pilot is usually playing for 30-35 minutes.
4. Join the extra-league and everybody will be happy :-P

Some people don't care about the money

They just want players's exp and exp for themselves - thats what mixed cup provide - 50% more.

Hope I explained clear