Economics is fun. Especially the law of supply and demand. The supply of tickets to the game is limited, ie the number of seats at a stadium. The demand for tickets is quite flexible: who is playing, when are they playing, why are they playing.
The reason that matches in other countries (the Super Bowl, the UEFA Champions League Grand Final, the World Cup Finals, etc.) can be so expensive is because the demand for tickets can far outweigh the supply. If demand exceeds supply, it can be quite profitable to raise prices as high as you can.
If the NRL is not getting the crowd or revenue it wants, it is presumably overcharging for the tickets. I would assume the NRL wants a full house. If they do, they need to lower prices so that more than enough people will be encouraged to attend. If they just want to maximize revenue, then lowering prices would help too. More people at a lower price can more more money than fewer people at a higher price.
Friday's game has no bearing on what preliminary finals were worth a year ago or next year or what any final was worth a week ago or 52 weeks from now. No one here is predicting a full house because there are not enough Manly fans or interested neutrals to pay what the NRL is asking.