The cost-benefit table considers the (monetary) costs of a certain alternative and their probabilities.
In the given example, there is a 30% chance idea 1 will cost 1000, a 40% change it will cost 2000 and a 30% chance it will cost 3000. This results in an expected pay-off of (0.30*1000+0.40*3000_0.30*4000)-(0.30*1000+0.40*2000+0.30*3000) = 700.
To use this technique, enough quantitative data should be already available.
Executing the Method
List Cost and Benefits
For each alternative, list the costs and benefits and the probability an alternative will cost a certain value and reap a certain benefit.
Multiply and Sum
For each alternative, multiple the probabilities per cost and benefit, and sum these values to calculate the expected payoff.
The alternative with the highest pay-off is usually the most attractive.