VPN has two cost components, 1. The number of hours the VPN gateway runs on Azure – typically this is 24x7 in most hybrid scenarios. 2. The amount of data that is transmitted across this gateway – Bandwidth.
Express Route has two variants – Metered and Unlimited. Each of these offerings has a port speed cost component, varying with port speed. The Metered variant has a bandwidth cost component on top of the port cost. The unlimited variant does not have a bandwidth component as the cost is included in the port speed component. Refer to the pricing page here for more details