They're probably one of the easiest cakes to bake and even the fancy ones cost about a fiver tops to make, but people are paying £20+ to have them delivered
They cost more me more than a fiver in ingredients, especially they are using more expensive branded chocolate that many people like to order. Mine are 38% chocolate and I use only butter and free range eggs. And yes, they are quite straightforward once you have done them a number of times and can get them right everytime. But they are a knack. People that sell their baking obviously have to be registered which can be a time consuming process.
In terms of making them to sell, the whole process generally is quite time consuming and you have to factor in the cost of equipment and packaging materials as well as ingredients. Shopping, preparing and baking, washing up, packing, getting them to the post office. £5.57 for first class post. The time taken to get the finished product to someone's door is probably much more than you think. There is much more to it than adding up the cost of the raw ingredients. Then there are all the normal costs involved in running a business.
By all means, make them yourself. But many people don't have the confidence, skill or time. So, like many other people, they buy it in from other people. Why do people go to restaurants when it's so much more expensive than cooking the same thing themselves?
Most of the brownies that are ordered from me are gifts. Particularly during this time, people are wanting to buy an experience for their friends, that's why they are happy to pay what you would consider a premium for a lovely handmade cake gift to turn up at their door.