At Sabernomics, JC Bradbury crunches some numbers on home run numbers for Alex Rodriguez during the seasons in which he admits steroid use:

So, what were A-Rod’s steroids worth? 2.37 home runs over two seasons, or a little over one home run a season. At least, that is the estimate based on the method I laid out above; however, it’s probably best to say that there was no observed effect.

In the comments section, Bradbury crunches the walk numbers to control for the possibility that a more powerful A-Rod was less selective at the plate and, again, finds no observable effect. There are some moderately outlandish hypotheses that could account for this, such as the league’s pitchers cycling steroids coincident with Rodriguez, so that a roided-up A-Rod would hit against roided-up pitchers and a clean A-Rod would hit against clean pitchers, but, well, Occam’s Razor.