Home charging is the cheapest way to charge. It’s the same as your domestic electricity, typically a quarter of the price or petrol or diesel fuel.
Charging mid-journey costs vary significantly, much more than petrol or diesel do. Many places, such as supermarkets and pubs, will tend to provide free charging to gain your custom. As ever, motorway services can be expected to be the most expensive. However, there is no need to fully charge and, given how long the last 20% takes and how most can charge at home, there is no need to.