Forget the cash!!
Cash only comes into if you plan on getting rid of the guy(ie first year players, players returning from injury)
And when to buy him?. Are you trying to win overall? If so, get him from day one, he is the best ruck in the comp and gives you the best chance. If you are just trying to win your league, then it might pay for you to wait and pick him up later.
If you buy him, chances are you will not be getting rid of him at any time(bar injury), so I don't think his price matters. It is all about the points return. Focus on the points. (I know, easier said than done considering the price) If you are confident he can get you 100 points per week, then get him, and forget about him losing money, there are no prizes for the biggest increase in team value.
Will I get him, not at the beginning. I can see a bit more value in the ruck department this year, so will be using cash elsewhere, upgrading later, a gamble, I know.