I will say this. There are people who would with your opinion of Boyd and Milford being overpaid. I think milf was paid on promise and Boyd on past performance. That said they are both good players, and despite criticism of their form (sometimes fair, sometimes not) are easily the best for their positions in the club, injury allowing.
Gillet though has been and still is if not the best 2nd rower in the competition, close to it. You may have a dissenting opinion based on recent performances, but remember he was essentially playing with a broken neck.
On his day, when fit, will run as hard as sua and make as many tackle as macca. Ones that hurt their opponents. Also he plays the full 80 minutes without break, allowing impact players to rest up.
You never know how players recover from injuries and Gillet may never be that player again but if he is close to it, he is easily worth his price.
As for Oates being worth more than Milford, let's break that down. Milford is a devastating runner who can draw defenders, offload in tackles, organise the attack and kick profficently. His form is inconsistent and needs to be corrected but in his worst games still has more impact on the teams attack than Oates because of the threat he represents.
Oates is a good winger in hot form, who deserves more money. He is a good runner, a proficient catch and an excellent finisher, but that's it. He can tackle alright on the wing but is sometimes caught out when he is closer to the ruck. He rarely passes or kicks (it's not his job). He almost never passes except to beat the last defender and has stuffed that up at times. No way would I lose Milford for him, let alone pay him more.