Without knowing anything else about Dan's roster, it sure sounds like it is.
There are two ways to analyze the potential gain from a trade like this: from a categorical perspective and from a valuation perspective.
Categorical: Cordero has 19 saves and is on pace to save 30. This would give Dan the four-point jump in saves that he's looking for. Even if Cordero were on pace to save 40, I would still use a 30-save pace to be on the safe side.
I don't know where Dan is in wins, ERA, WHIP and strikeouts. Here is the ZIPS updated projection prorated for the rest of the season (absent saves).
Billingsley: 5 W, 72.2 IP, 69 H, 28 BB, 30 ER, 65 K, 1.343 WHIP, 3.74 ERA
Cordero: 2 W, 23.9 IP, 18 H, 9 BB, 8 ER, 18 K, 1.130 WHIP, 3.01 ERA
It looks like Dan should also gain a fair amount in WHIP and a little bit in ERA. However, it looks like wins and strikeouts tell the story for Dan on whether or not he should make this trade. Depending on where Dan is in the standings, losing 47 strikeouts and three wins could mean nothing or could mean everything. In my N.L. home league, those 47 strikeouts could lose Dan one point...or four, depending on where he was sitting.
Valuation: Categories matter more this time of year than valuation. I'm far more likely to use this as a yardstick in May. If a trade looks neutral to me categorically but I'm gaining $5-10 of potential value, then I'll probably make the trade since I'm getting more chips to trade later.
In this case, Cordero and Billingsley's projected ROS stats look like a wash. Those numbers above are worth $6 for Cordero the rest of the way and $5 for Billingsley. This impact is negligible and would not sway my analysis.
In general, an elite starting pitcher > an elite closer and a second-tier starting pitcher = an elite closer. Sixteen National League starting pitchers cracked the $20 barrier last season; only four N.L. closers pulled the same feat. Billingsley probably is closer to the third-tier than the second-tier, but Cordero isn't an elite closer. From a valuation standpoint, this trade seems fair to me. If I could make the big jump in saves without getting killed in strikeouts, this is a deal I would make.