To start, as with any good model, we need to lay out some assumptions. In this case, I am going to assume that your league is like the original ottoneu league (prizes for the top three finishers) and that you are like me (finishing in the money and bragging rights are your main objectives, but you place a serious premium on finishing first, beyond just the additional cash). I am also quite content to finish dead last if it means putting me in a better position for the following season, and I am similarly happy to doom myself to a dead last finish next year in order to win this year - not everyone will make these trade offs, so you may need to adjust the criteria accordingly.
With that said, here are my three criteria for determining if you should buy.
1) You can't win the league with what you have. It's this first criteria that some people (read: I) tend to ignore.
In 2007, I basically ran away with the original ottoneu league. I led more or less wire-to-wire and had a double-digit lead most of the time. But on August 7, 2007, I made not one but two trades. In total, I added Pat Burrell, Jonathon Broxton, and Brandon Webb in exchange for Ian Snell, Carlos Villanueva, Ryan Dempster, Carlos Gomez, and Matt Murton. Broxton, Burrell and Webb were all way to expensive to keep, at the time, so this was purely a win-now set of moves.
It happens that in this situation I had fortune on my side. In hindsight, we all know that what I gave up amounts to...well...it doesn't amount to anything, really. Dempster would be nice to have, but the rest of those guys really have not been missed at all. But imagine Murton turned into the high OBP, solid OF I expected? Or that Snell didn't fall off a cliff or that Gomez developed... I potentially would be out an awful lot of cheap talent and all I would have to show for it was a title and a check that I would have won even without the trades.
Cheap, young talent is the lifeblood of a successful ottoneu team - don't give it up to secure a victory that was, barring something unforeseen, already secure. You have to be realistic, but when deciding to buy, make sure you are buying something of value - if you can win with what you have, don't mortgage the future.
2) The chance to win is in front of you. When this happens, jump at it. MLB teams rarely get a shot to win a World Series (Yankees and Red Sox notwithstanding) which is why, when the chance arrives, very few prospects are untouchable. In the immortal words of Herm Edwards, "You play to win the game," and you don't pass up that chance to win.
I am currently sitting 4th in the FanGraphs Experts League and have a pretty talented farm system (Hosmer, Montero, Moustakas, Teheran, Lamb). My expectation is that 2012 is my year, since those guys should be producing, and I will still have an inexpensive rotation (Hellickson, Weaver, and Marcum are all cheap enough to keep) and some potential value pieces on offense (Utley, Domonic Brown, Arencibia, etc.). But let's say we roll into June or July or August and I have a shot to win right now. Maybe the lineup I have can't cut it but if I gut my farm system to add a couple pieces, I can take the league. My stance is that I HAVE to go for it. Yes, I am dooming my team to failure in 2012, but I will fail just as badly if Hosmer turns out to be a AAAA guy, Lamb doesn't develop and Hellickson blows out his arm. Why go into 2012 on a gamble if I can make a much safer bet on 2011?
3) There are sellers out there. There is an odd clash between buyers and sellers in ottoneu leagues - the earlier you buy, the better (you give up future value either way, but the difference between four months of an all-star and two months of that same player is huge), and conversely, the later you sell, the better (you maximize the time you have to see if your team can turn it around and compete).
Looking at this from an economic standpoint, to entice a team to sell earlier than they want, you have to compensate them for the option value they would have if they had sold later. Effectively, what this means is that there are ALWAYS sellers, as long as you offer a high enough price.
This is where things come down to personal choice. I believe in buying aggressively when you buy, and if you have to give up one more prospect or a better pair of prospects or something to get a deal done in April or May instead of July or August, you are probably better off paying that price.
But I know that a lot of other owners feel differently - in their minds, it seems, if you are not 100% sure you can win, you should wait to make the trade. The problem I see with this is that the value you need doesn't change, but I think the TOTAL price you have to pay goes up. Yes, you can probably get Albert Pujols for less on July 15 than on May 1. But five months of Pujols is far more valuable and to get the same value on July 15, you probably need to trade for Pujols and another star-level bat. So if you need those five months of value to win the league, your choices are to pay a premium to get Pujols ASAP or pay for Pujols and, for example, Braun, in a couple months. But my guess is that Braun and Pujols on July 15 will cost you more than Pujols alone on May 1 (as long as Pujols owner is willing to sell him on May 1).
Finally, ONLY get that player from an owner who has shown a willingness to sell (even if they are hesitant). If you have to talk an owner who is not considering selling into selling, you will overpay. But, as soon as you get the sense that an owner is ready to move present pieces for future ones...jump on it.
That was all a bit complicated, so let me boil it down to this: if you are a player or two away from winning, you should get that player as early as you possibly can. The longer you wait, the less value you get per trade, the more trades you have to make and the higher the total cost.
So where does that leave you, an owner who thinks you might be a buyer, today? Think realistically about your team - can your team as currently constructed "win" (with win being defined as whatever you want - first place, top three, competitive all year, etc.)? If yes, don't waste future talent for unneeded pieces. If not, consider whether you can win AT ALL. If not, again, don't waste your time buying and chasing an impossible dream. If yes, look around at the market - are there sellers out there? At least people who could reasonably be talked into selling? If no, then hold off - don't go crazy overpaying to convince someone to sell. But if there are is an owner who shows a willingness, however slight, to sell off pieces, consider that a door left ajar and find your way in. You'll get a better return from one big trade today than from needing to make two or three trades in August.