The cost to pick a player in the Rule 5 draft is $50,000. It is unknown how much the Dodgers paid the Mets, but it was almost certainly at least enough to cover the fee plus, as Carl Spackler might say, a little something extra for the effort. But for some idea of the trade:
Sandy Alderson joked the Mets added a crisp $5 bill by making a Rule 5 selection and trading Seth Rosin to another team.— Adam Rubin (@AdamRubinESPN) December 12, 2013
"Who's on a $5 bill? That would be the player to be named later." -- Sandy Alderson.— Adam Rubin (@AdamRubinESPN) December 12, 2013
Rosin, 25, was 9-6 with a 4.33 ERA in Double-A Lehigh Valley in 26 games in 2013, including 23 starts, with 96 strikeouts and 35 walks in 126⅔ innings. The 6'6" right-hander was drafted in the fourth round out of the University of Minnesota in 2010 by the Giants, and was sent to Philadelphia in 2012 as part of the Hunter Pence trade.
The Dodgers would have to keep Rosin on the active roster for all of 2014 (or disabled list, though Rosin would need to be active for at least 90 days), and should they want to remove him from the roster (i.e. to send him down to the minors) would first have to offer him back to the Phillies for $25,000, half of the fee for drafting him in the Rule 5 draft.
While it's highly unlikely Rosin would stick on the Dodgers roster solely as a starter - though perhaps he could fill the otherwise vacant swing man role - he has had more success as a reliever in his four minor league seasons. In relief, Rosin has struck out 32.0% and walked 8.3% of his batters faced with a 3.47 ERA, compared to 18.3% and 6.8% as a starter with a 4.23 ERA.
John Sickels at Minor League Ball has this scouting report of Rosin:
He has a low-90s fastball, a curveball, and a change-up. He did not dominate as a starting pitcher but he usually throws harder in shorter stretches and had some success when used as a reliever in the Giants system, which will probably be his long-term role in the majors.
The last time the Mets drafted a player in the Rule 5 draft from the Phillies then traded him to Los Angeles, the Dodgers ended up with Carlos Monasterios four years ago.
With Rosin, the Dodgers have 36 players on the 40-man roster.