While there's no formula (that I know of) to ensure a bargain, there are certain classes of players I've found are more likely to come at a discount disproportionate to their risk/reward profile. In no particular order:
Post-hype prospects
These are players who were once considered can't miss stars that didn't yet pan out as expected. They burned a lot of owners who gambled on them, and the buzz has worn off as a result. While everyone's chasing Buster Posey, Carlos Santana and even Jesus Montero, who could be the next star catcher, I'm looking at Matt Wieters who's guaranteed at-bats and won't turn 25 until May. Alex Gordon was once a top-three prospect in all of baseball, but injuries, inconsistent opportunities and a position switch derailed his development. He just turned 27 and annihilated Triple-A last year with a 1.019 OPS.
Homer Bailey seems like he's been around a long time, but he doesn't turn 25 until May 3 and struck out 100 batters in 109 IP last year. Plus, Albert Pujols singled him out for having particularly nasty stuff last year on our Sirius XM show. Even Rick Porcello's 4.65 K/9 last year won't deter me from gambling on his talent this year.
Last year's bums
I wrote extensively about this class on the RotoWire blog, defining it as players who are mostly healthy, reasonably assured of playing time and who underperformed expectations a year ago for whatever reason. I almost think you could invest blindly in these guys, i.e., just bid $1 more than the room for as many of them as your budget allows, and you'd probably finish in the money.
Last year's one-hit wonders
The year after Cliff Lee's 2008 Cy Young award, I bought him in AL LABR for just $16. How was that possible? Because Lee had come out of nowhere (He was a reserve pick in 12-team AL LABR that year), no one believed he could possibly repeat. This year Jose Bautista, Colby Lewis, R.A. Dickey and John Axford could actually be bargains in savvier leagues.
Older players
Take a look at Scott Rolen's first half last year - .290/.361/.548 with 17 homers. When a borderline Hall of Famer in his mid-30s stays healthy for any stretch, don't be surprised if he produces more than many players in their primes. Vlad Guerrero and Magglio Ordonez (before he got hurt) also had good years. This year, I expect Manny Ramirez (39 in May), Bobby Abreu (37 in March), Jorge Posada (39), Alex Rodriguez (35), Chipper Jones (39 in April), Lance Berkman (35), Alfonso Soriano (35) and Ichiro (37) collectively to outearn their draft-day prices.
Injury prone but not injured
Players like Chase Utley, who's talking about "the big picture", i.e., his "career," regarding his current knee injury that's not responding to cortisone shots, and Joe Mauer, who's had injections to lubricate his knee following offseason surgery, worry me. Grady Sizemore, coming off microfracture surgery and unlikely to be ready for Opening Day, is even worse. But despite missing significant time three of the last four years, Ian Kinsler (two years removed from a 30-30 season in 144 games) is completely healthy, and in my opinion overly discounted. Same with Rickie Weeks who went for just $20 in NL LABR even though he played 160 games last year. Aramis Ramirez ($18) seems to have strained every muscle in his body at one time or another, but he had a strong second half (.276/.321/.526) and is healthy heading into the year.