Maybe the answer is started in this case study. The Cows have put three years development into him. Once a player is on an NRL contract he’s free game but should be compensated for the three years development. Again this argument I’m putting forward will probably be able to easily be picked apart and that’s fine but off the top of my head:
A player needs to be compensated up to the age of 19/20. If a club poaches a player at 20/21, so be it (this could easily be adjusted / re-visited).
So if a club picks up a player at 15 or younger let’s say, then the kid is essentially that clubs junior (again this will likely be easily argued but just go with it). Club A (in this case Cows let’s say) put three/four years development into player X. Club B (obviously in most cases the Roosters) comes along and poaches this kid before he turns 20.
Club B now has to pay Club A let’s say a compensation figure of $50k a year, so club A gets 200k (for the four years l) or 150k (for the three years) and they can add this to their salary cap and club B loses it off theirs. This is a strong disincentive to poach.
Additionally, on top of the transfer fee, for every year a player is at a club from say 16, they get a discount up to the age of 19 of 50k a year to a maximum of 200k as a discount in the salary cap.
There are likely holes left, right and centre in this but it rewards a club for development and retention and penalises a club for poaching, at least to some extent.
I would prefer it to work as cap relief.
In the Ponga case Cows would get some cap relief, which helps them to compete with the Knights paying overs or forces the knights to pay even more to poach away a player they haven't had any involvement in.
I believe it was reported that Knights got him for $500k so let's go with that... if 10% salary compensation is provided to Cows then they can match the Knights offer of $500k, but only ~$450k or so is seen on the cap. If they go the full hog and match the $500k on the cap then they can offer Ponga up to $550k to stay and not be worse off than the knight's cap.
If Ponga still chooses to go to the knights then his salary cap relief resets back to zero i.e. he would have to be at the knights for x years before they can then get their 10% salary cap relief. It can start to become an incentive for one club players again.
If you move somewhere at 20 or so your cap relief opportunities reset... so a guy like Carrigan who will be in like his 5th or 6th year of NRL by the time this contract expires could be getting 15-20% cap relief from the broncos vs Dfifi who moved away and now has to reset at the titans. It allows us to compete with vultures trying to lure players away when they've become household names... and say after 10yr it could become 50% salary cap relief, for someone like Lockyer we could have been offering him $1m and only $500k would be on the books... he's not moving to another club, but him staying at the broncos for so long gets rewarded by the player and the team.
They would have to find those sweet spots for bumps in the salary cap relief because 50% cap relief across the board is a huge bump on the salary cap... but even capping it at 20% ends up being $200k relief on a $1m salary and if somehow every player on your squad was able to fall into that 20% bracket then that is only $2m extra on the current $10m salary cap, which isn't all that much across a 30 player squad.
Transfer fees work in soccer, because it's basically a trade system. The players moving around are all under contract when they move. The next Ronaldo wants his pay day, but is stuck in the Portuguese league for the next 3yrs. Real Madrid come in and say we'll give you $20m to break that contract and we can take him now. It becomes a viable way for club's to make money and keep themselves afloat. Dortmund is basically a development club... young players sign with them knowing they will get good game time in a good team and then be allowed to move on in a few years.
In NRL most players are moving when they are out of contract or they're forcing moves mid contract. I would prefer a trade system to deal with mid-contract moves, but transfer fees can't work for a player looking to move when he's out of contract... in soccer the clubs get zero remuneration if a player leaves once their contract expires, which is the only way it can work.
What if 18yr old Joe Blog has zero chance of making grade but a team wants to give him a chance when his contract expires. Forcing the next team to pay a concession on a player you had no intention on using could prevent Joe from ever playing and that would be illegal.
I think the transfer fee only works if a player wants to leave mid-contract... like Walsh wanted last year. Warriors and Walsh want to start early... cool give us some cash and you can have him... but we're not talking Real Madrid here, the transfer fees in NRL land would be a couple 100k at the absolute max.