Econo is an automatic function of the PCB:NG system. If your design may qualify for Econo, you'll be notified in the interface:

Note that pricing will not change until your order has qualified for Econo. (Econo requirements here)

Need more info? Hover over the notification to get the basics:

Or click on the notification for detailed info, including pricing:

Pricing will update after part placement/verification in the Placement Window, and will be reflected in the Order Tab before you place your order: