In this relatively simple model, service levels were fairly easy to determine. The circuit was either up or it was down. Fast-forward to the late 1990s. Frames and cells are everywhere. Today, there are many different shades of gray when it comes to a service being active or not.
Latency can vary considerably, as can bandwidth utilization.
Applications may or may not work. If the service levels asked for by the user are not met, then the users pays a pro-rated fee. Unfortunately, many managers have begun using SLAs as a mechanism for determining monetary compensation only and are even willing to put up with degraded service if they can essentially get a discount on it. Thus, I would argue that you should use SLAs not to determine the cost of a particular level of service, but to determine at what threshold to demand the activation of backup service when the main connections degrade. It may not be popular with the service providers, but it sure will work.