In lighter demand scenarios cost will be lower, but in high demand scenarios cost can still be lower since half of the average developer's day is spent waiting or executing lower value administrative tasks, while only 50% of their time goes into true development activities.
When total demand is low and only a few resources are financially justified, development must commonly be approached in a serial fashion. The team approach of an SDaaS model is able to provide both of these needs in parallel without slowing down work or impacting overall costs.
There may be an inclination to think there is a lessening of the importance or role of the developer in an "as-a-Service" model. This is not the case. The need to have a high level of programming skills, as well as effective communication, collaboration and self-management skills are as important in an SDaaS model as a resource-based model.