Loyalty to broadband providers means households could be up to 90% a year more than new customers, a watchdog warns.
A Which? Report found to have been with the same firm at least three years.
This is the case for nearly nine in 10 TalkTalk customers, the most loyal of the 12 major suppliers polled. The average rise between its introductory offers and standard tariffs is 15%.
But its Faster Fiber Speed Boost leaps 54% from £ 25 a month to £ 38.50 after 18 months – £ 162 extra a year.
Eight in 10 with BT have stayed put for at least three years, despite an average of 23% price rise when deals end.
BT's standard package soars 32% from £ 24.99 to £ 32.99 18 months, equal to £ 96 extra a year. The biggest hike was Post Offices Unlimited Broadband, up 89% from £ 15.90 to £ 30 after 12 months – some £ 169 extra a year.
Alex Neill of Which? said: "Suppliers should be hard for your loyalty."
TalkTalk said: "We're committed to treating all customers fairly. We give notice before and after contracts end. We reward loyalty by giving great deals. "
BT said: "We fully agree that customers should not overpay. This is about clear. "BT added it is very clear what the price will be when [a deal] ends ".
The Post Office said it offered "the UK's lowest-priced unlimited broadband", adding: "We clearly signpost out-of-contract prices at point of sale."