I got a fair bit of criticism from veteran AVForums members because a much larger proportion of competition winners are women than the usual forum population.
We surmised that lots of women register to enter competitions and have no interest in contributing to the forums in any other way.
This is the main reason why I specified a usergroup permission to enter competitions.
So, my advice would be to, of course, try and run as many competitions as possible.
Have some for lower value prizes open to new members to encourage people to register. You always want people to register
.
But reserve the big ticket prizes for those people in the next usergroup up the ladder. That way you don't annoy your veteran members.
Incidentally to get from new member to member on AVForums, people will have to earn 3 positive post ratings. So perhaps you will get your competition prize chasers to contribute to your forum after all.
I'm actually planning on having 4 tiers of membership with competitions exclusive to the higher levels also.
It means less entrants, but the competition sponsors get the same level of publicity on the home page sidebar and in the competition list.