If your game requires any any items or fee in order to participate, then it would be against the game rules.
Rules of Runescape: Unofficial Player Run Games Of Chance - You must not advertise, organise, promote or take part in any unofficial player run games of chance where in game money or items are staked on the outcome of a random in game activity.
Additionally, if you are considering advertising in the Events and Competitions Forum, make sure that your 'game' complies with the Events and Competitions Rules: "No giveaway-only threads are allowed here. This means no drop parties, no in-game giveaways are to be advertised here. You may advertise your event and give prizes away as part of that event, but due to the high number of scams and no-show hosts you may not post solely about any prize giveaways, and you must make no mention of what prizes will be available. To be clear, you may post that prizes are being given away, but you can't state what they are and your event must be about another activity, not just the giving and receiving of wealth! "