Every auction will have its own market. You need to have a sense of every player's average auction value, but you also have to adjust to the circumstances of your particular auction. Once the market has been established, something that will happen within the first few minutes, you'll want to do what you can to make sure no one in your league gets too great a bargain. You can't protect against this all the time, nor would you want to. It's not up to you to be a one-man police force, and you don't need to take any unnecessary risks. Still, every dollar is crucial, and forcing someone to spend $34 on Dez Bryant instead of $30 could make a huge difference later in the auction.
The above principle is known as price enforcing, and while you shouldn't need to engage in it more than a few times during an auction, you should familiarize yourself with the concept so you'll know when to use it. Before price enforcing, there are three questions you should ask yourself. If the answer to any of the three is "no," let it go. If, on the other hand, you answer "yes" to all three questions, enforce away.
Question No. 1: Would I be happy with this player on my team?
Any time you enforce your auction's established market value, you put your neck on the line. That's because you very well could end up with the winning bid on the player. There is no guarantee that whomever you're bidding against will continue to go dollar-for-dollar with you. Given that, you have to be comfortable having the player in question as part of your roster. It doesn't get much worse than spending a decent chunk of change on a player you don't even want simply because you felt a duty to be the tough cop at the table. If you only get involved in price enforcing on players you like, though, it's a win-win situation. Either you end up with the guy, or you force someone in your league to pay more for him.
Question No. 2: Is this truly a below-market price?
For price enforcing to work in a specific situation, the bidding for the player has to be running below market value. That's why no one should think about price enforcing until a fair number of players have been purchased, allowing the market to set. Once you have a few data points at a position, you can understand the market and begin loosely pricing the players yet to be nominated. For example, let's say the first three backs nominated are Adrian Peterson, Trent Richardson and Frank Gore. Peterson goes for $60, Richardson comes in at $42 and Gore sells for $28. (This is a clean example since the three are in different tiers, but it gives you an idea of where many backs should slot in the auction.) The next back nominated is C.J. Spiller. You may have strong feelings about the Bills runner, but most people would rank him between Peterson and Richardson, though closer in dollar value to the latter. If the bidding stalls in the high $30s, this is a perfect chance to jump in with a price enforcement. Once it creeps past Richardson's $42, you've hit Spiller's expected value based on the running back market price. At that point, whether or not you stay in the bidding depends on how much you like him.
Question No. 3: Can I afford this?
OK, you've answered yes to the first two questions. You'd like Player X on your team, and the bidding is below market value. There's just one more box you need to check, but it's an important one. Do I have room for this guy in my budget and on my roster? Just because someone is selling for less than the going rate doesn't mean he'd be a fit on your team. You still have to consider the context, even when there's a bargain to be had. Let's use the example in Question 2. You'll recall that we had Peterson going for $60 and Richardson for $42. Well, suppose you bought both of them. You've already spent $102 -- more than half a standard $200 auction budget -- on running backs. Can you really afford to spend another $38 or $39 on a back, even if you think it's a great deal for Spiller? Remember Question No. 1. You have to be comfortable with adding the current player to your team. Also recall one of the general maxims of any auction: You must know your roster and how much money you have left at all times. There's a chance you'll have to let a bargain and the opportunity to price enforce pass if your roster and/or budget precludes you from going after a certain player. It may hurt at the time, but you'll thank yourself when you don't have to fill key positions with a bunch of single-digit players.