TAshahin the site already runs official Chess.com tournaments each month for people in different rating groups. We did consider adding prizes for these (a long long time ago) but the sad problem there was that some may be tempted to cheat in order to win the prize.
There are some good natured members who create tournaments themselves and will offer a free membership for the winner though but not very often.
Hi dear masters ! whats your idea about holding up different competitions for different ratings ?! I think chess.com can hold up these competitions and contemplate rewards for winners !! For example for first : one month diamond premume and etc . I wanna know your ideas