Former students Andy Weinbach (now teaching at Coastal Carolina) and Mark Wilson (St. Bonaventure) were discussing this with me. Their assessment, derived from market prices: pretty small. Here's their take:
How valuable is Brett Favre to the New York Jets?
The Jets acquired Brett Favre, believing their team would be improved, but what do the markets reveal about this move? For proposition bettors wagering on the number of regular season wins by the Jets, the move seems to be an improvement.
Betting markets can provide insight into how much the Jets have improved, but the answer is unfortunately not straightforward. Many books temporarily took the Jets regular season wins proposition off the board in reaction to the Favre acquisition. Bodog sports currently offers the Jets at 8 wins (-155o),(+125u). If this is assumed to be an unbiased forecast, (which is a reasonable, but not necessarily accurate assumption), using some statistical wrangling, and ignoring pushes, assuming all money will be refunded in the case of a tie, this translates to a roughly 57.8% chance that the Jets will win more than 8 games and a 42.2% chance that they will win fewer (Again, this assumes ties are not possible. While ties are indeed possible, they should be revenue neutral for bettors).
Unfortunately, we don’t have the Bodog price from July. However, a reliable source tells me that a local shop had the Jets priced at 7 wins (-150o),(+120u) on July 22nd, when Pennington was still the expected starter. As of August 10, that same shop offered the Jets at 8 ½ wins (-115o),(-115u). If we ignore the possibility of a tie, the July 22 price at this shop would suggest a 56.9% chance of winning more than 7 wins, and a 43.1% chance of winning fewer. The current price suggests a 50% chance of 9 or more wins, and 50% chance of 8 or fewer.
While these figures are not directly comparable, a loose and perhaps fair interpretation might be that the acquisition of Brett Favre is expected to return one additional win for the Jets this season.
Meanwhile, in the NFL.com fantasy league, www.nfl.com/fantasy/rankings/wr current rankings indicate a recent increase in value for Jets receivers Laveranues Coles and Jerricho Cotchery, suggesting fantasy players believe Favre will have a positive impact on the two wide receivers.
The same reliable source reports that the local shop has priced in slightly lowered expectations for Miami (Pennington’s new team) since July. In July, the price was 5.5 wins (-125u) and (-105o), but has now moved to 5.5 wins (-130u) and even.
Obviously, one additional expected win at this stage might represent a significant boost to a team's playoff chances. That would be another calculation worth making. If you know or can quickly determine the answer, do leave it in the comments.