How much this will cost depends on how many files the portal pins and how fast and often it needs to access content on Skynet. If you're running your own portal, the cost and necessary hardware can be minimal. That said, running a public web portal, like running any server, has other costs associated with it as well. To service hundreds of users all storing and accessing data on the Sia network, the portal will require fast internet, powerful hardware, and some love and care in the form of maintenance. Running a web portal isn't always cheap, and portals may ask users to help to cover these operating costs.