A club can demand a fee if the player is under 24, I think!
Over that age they can leave free at the end of their contract under the Bosman rule. Hence you got a fee for Westerman but not Shenton.
The only way you could keep your players if the RFL/SL adopted the system being trialed by the Aussies next year which they hope to replace the salary cap. It's a points based system with a team 17 allowed no more than say 100 points. The Aussies haven't worked out the full details themselves yet!
It's runs something like, a home grown player who is an International (Shenton) will have a maximum of four points.
An International at a club who is not home grown will be nine points, Shenton at Saints.
A lad like Milner on his debut maybe would be one point until he's been f/t pro for two years, then two points. If he moved to another club he'd be far more.
This maybe would work here but RL/SL have a problem with a club like yours who continually bring players through but can't keep the best of them. Perhaps when you get your new stadium and far larger crowds plus new revenue streams then you will be better fixed to keep your lads.
It really doesn't work in anyone's favour when clubs can bring lads through and can't keep them. As a Wigan fan, we had seven lads in the U18s academy team that beat the Aussies. Most if not all will stay at Wigan, although Sale RU are always on the prowl. Milner has impressed everyone who saw the game, a real talent. What are the chances of him staying with you though? At present not much. If you could up your revenue and have a chance of winning things then he'd probably stay.
I wish you the best of luck with that one.
