The single loss expectancy (SLE) is a measure of the monetary loss associated with a single occurrence of a risk. The SLE can be calculated by multiplying the asset value (AV) by the exposure factor (EF), which is the percentage of loss that the asset would suffer if the risk occurred. In this case, the asset value is the revenue generated by the service, which is $1,000. The exposure factor is the impact of the attack on the server's capacity, which is 20%. Therefore, the SLE is $1,000 x 0.2 = $2001.