CTR 4 times higher on 1st results vs. 2nd.
Here's an interesting observation. One one of my affiliate sites that perform well the CTR is 15% when it appears 1st in G's SERPs. When my site appears 2nd, the CTR is 4-5%. This is consistent with the top 5 keyword phrases.
Conclusion: If you could spend twice as much to get to the first position when your at the second then do it. You'll still get 4 times the traffic for only twice the cost. This may apply to AdWords as well (not sure).
This is probably not true for other positions (>3rd), but I suspect the traffic CTR vs. position in Googles SERPS are not linear.