Comcast Xfinity Internet Cost. Typically, the cheapest comcast xfinity internet plan is its 50 mbps plan. Up to 4 devices at a time. Up to 8 devices at a time · save up to $20/mo for 24 mos with xfinity mobile. Xfinity internet plans, prices, and speeds ; Up to 50 mbps download speed ; Comcast is a giant in the cable tv industry and like its largest competitors, at&t and charter communications, it has continued to maintain its large subscriber base by changing with the times. How much does xfinity cost?