The cheapest charging usually happens away from motorways, at places where electricity isn't the business being sold: supermarket car parks, retail centres, workplaces, and your own home if you can charge there. Motorway service areas sit at the other end, because they combine the highest demand with the least competition.
Why supermarket and retail charging tends to be cheap
A retailer that installs chargers in its car park isn't trying to make money on electricity. It's trying to keep you in the car park long enough to shop. That incentive pushes the price down, sometimes to free, because the retailer profits from what you buy inside, not from what you charge outside.
Why motorway charging tends to be expensive
A service area operator faces the opposite incentives. Drivers there need power quickly, have nowhere else nearby to go, and won't wait around browsing a shop for an hour. Fast DC chargers also cost more to install and run than slower AC ones: a bigger grid connection, cooling systems, more maintenance. Both of those pressures, captive demand and higher equipment cost, push motorway prices toward the top of the range.
Working out whether a detour is worth it
Cheaper electricity somewhere else only helps if getting there doesn't cost you more than it saves. Do the arithmetic before you commit to a detour: multiply the kWh you plan to charge by the price difference between the two stations. Charging 30 kWh at a station that's €0.10 per kWh cheaper saves you €3. If reaching it costs you an extra 15 minutes of driving, ask yourself honestly whether 15 minutes is worth €3 to you today. On a short weekday errand, probably not. On a long trip where you're stopping anyway, the detour often costs you nothing extra at all.
Timing around motorway exits
Prices often step down just before or after a motorway exit, where a station is competing with ordinary local traffic instead of captive drivers who have no other option. Pulling off a few minutes early or continuing a few minutes past the interchange, rather than charging inside the service area itself, can put you next to that lower local price instead of the premium one.
Building the habit
None of this requires memorising a single average price, because averages hide exactly the variation that matters here: the same 30 kWh can cost you noticeably different amounts a few hundred metres apart. Comparing station by station before you commit, rather than trusting a location's reputation for being cheap or expensive, is what actually saves money over a year of charging. That comparison is what goodcharge is built to do automatically, station by station, at the moment you need it.