A subscription model is considered an upgrade as it is not part of the Starter Site standard features. It depends on the type and complexity of the subscription model you’re wanting to implement. The most common subscription models are 3-tier models or an annual / monthly model. Subscription models can range anywhere from $500 - $2,000 depending on the complexity.
The special discount code section is a customization option to include within a Subscription model that allows users to enter a special discount code. This discount code would be generated within the marketplace, however, that is a detail that would need to be designed as part of the subscription model.