This topic has been covered extensively elsewhere (most notably on FanGraphs.com: "Chris Young's fatal flaw"). Dave Cameron neatly sums up his article: "[q]uite simply, Young is worse at holding runners than anyone else in baseball is at any other skill."
Of course, many great pitchers have been notoriously poor at holding runners. Greg Maddux, for his career, was stolen against at a 76.3% rate (league average usually varies between about 68-72%). Chris Young, for his career, is 91% (and 100% last year and so far this season). Statistical analyses show that the "break-even" point for steals sits around 75%, implying that teams generally deploy a sub-optimal stolen base strategy.
Back to Young. On April 27, the Rockies went 8-8 in stolen bases in Young's 3 innings. Dexter Fowler went 5-5. Watching the start, it was clear that Young wouldn't have lasted long, with or without the stolen bases. However, we can estimate exactly how many runs he cost the Padres during that start.
To start, familiarize yourself with the run expectancy matrix (data is from 1999-2002, but other studies have shown that these rates are acceptable for the modern era). Basically, this is MLB-wide data showing how many runs a team can expect to score in a general runners-out situation. There are more specific systems available that customize these on a team-by-team basis, but these are more difficult to use, and do not provide a significantly greater amount of accuracy.
Using the matrix linked above, we look at each stolen base against Young on his 27 April start. Additionally, I have pulled film from MLB.tv to look at each base and give a rough estimation of the Nick Hundley's chance at preventing the theft. I'll run through the first 2 stolen bases then will present the summary.
SB #1: Fowler on 1B, steals 2B, 0 out. With 0 out, runner on 1st, a team can expect to score .953 runs. With 0 out, runner on 2nd, that becomes 1.189 runs; essentially, the stolen base is "worth" the difference, or .236 runs.
SB #2 and #3: This was a double-steal. Fowler stole 3rd while Spilboroghs stole 2nd. 1st and 2nd with 0 outs expects 1.573 runs; 2nd and 3rd with 0 out expects 2.052 runs, a difference of .479 runs.
Summary data for all 8 SB against Chris Young: the expected cost was 1.61 runs. Of course, several times the Rockies would've scored those runs with or without the stolen bases, but it's fair to say that Young likely cost himself between 1-2 runs because of the 8 SB against him. In reviewing video, Hundley had no chance at 4 of the stolen bases. Twice he dropped the pitch, and twice could have gotten the runner with a perfect throw. I think it's fair to lay most of the blame at Young's feet: several times, the runners had a head start before C.Y. started his motion!
I have two thoughts to this: first, stolen bases generally aren't worth that much. The most valuable situation in this example (excluding double steals) was Fowler's 1 out steal of third; it "earned" the Rockies about .293 runs. Even in an extreme example, 8SB in 3IP only cost (at most) 2 runs. Secondly, I think that Young is so outrageously bad at holding runners, that an event with such a small effect on runs (the stolen base) will force him to focus on something other than pitching to the hitter.
For last season, runners were a perfect 44-44 in stolen bases against Chris Young. Assuming that the overall weighted run expectancy for the stolen base is about .20 (as has been shown elsewhere), Young cost himself almost 10 runs last year in stolen bases. 10 runs is about equal to 1 win; Young's value in 2007 (remember, 2008 was marred by horrific injuries) was about 4.5 wins. This suggests that Young gives away about a quarter of his positive value simply because he's unable to hold a runner.
For several years now, the Padres defense against the running game has been an absolute joke. Opposing teams were 150-176 (85.2%) in 2006, 189-209 (90.4%) in 2007, 268-208 (81.5%) in 2008, and are 25-29 this year (86.2%). Fortunately, opponents haven't yet begun to steal as much as they should against the Pads, but the question remains: how long will it take other teams to follow Clint Hurdle's lead and give the entire team the green light? The Padres are costing themselves 2-5 wins per year by not defending against the stolen base at a league average rate. FF wonders how long the Padres can afford to get dismal offensive as well as defensive production coming out of the catcher's spot...