Not sure I'd call it a "good portion" in this context, but it's 6.2% according to FHWA.
> Today, the 46,730-mile Interstate System includes approximately 2,900 miles of turnpikes.
> Today, the 46,730-mile Interstate System includes approximately 2,900 miles of turnpikes.
If you go from Philadelphia to Chicago, it's a toll-road the entire way (PA turnpike -> Ohio turnpike > Indiana tollroad -> Chicago). That's an outlier route, but toll roads aren't uncommon, especially in certain states.