He was offered $400k on his contract... this was in 2005.
He was on more than 10% of our salary cap, as it was 3.25 mil back then.
To put it into perspective, that is 1.2 million, if that same ratio was used along with today's salary cap. His issue was not with the club, it was with the NRL's maximum payment to a player.
Take a look at CNK from the Raiders. Bloke was signed on a bargain basement price, due to lack of demand by other clubs. Remember, demand sets your price in a fair market. He had a hand full of FG at this stage, and was stuck behind RTS. No one was really in for him, Raiders picked him up for a bargain, and he would probably be the best purchase of any player at any club this year, based on how much his contract was worth. Now he is worth more, but the Raiders, the very club which took a punt on a player who was not an established first grader, are reaping the benefits of a bargain, whilst he is reaping the benefits of increasing his potential for future contracts.
And also - using your example of "if someone offered me more for the same job" does not really hold well in this scenario. For starters, it is very rare for contracts to withhold you work for a competitor, within your contracted terms. Secondly - he broke his contract, in the most dog act of ways. At the very least, players in this day and age will make it known that they are unhappy, and make contact with the club. Whilst I still despise this type of action, it is what it is with this entitled generation.
Also, and very importantly - he stated that he did not just leave for money. He blamed the club culture, the people involved and was feeling very down-trodden about the clubs current state. At least that's directly from the horses mouth.
He walked out. 1 year into a 5 year deal. Leaving us in the lurk. Without a thought or a whisper. I am fully for players getting what they can, when they can. But at what point and at what time does the club get praised for good business? A contract is not one-sided, and if a club takes a punt on you, and it works out better for them, why are players then entitled to more? Let's flip your logic - if SBW signed for another club for $600k on a 4 year deal, and they reduce his pay for the remaining seasons because he under performed in his first season. Is that fair? The point here is the club will pay you, regardless of what happens to you as a player. The CNK is a perfect example of that. If everyone thought the same way you did, and behaved the same way, then there would be no point in contracts.