I think that you underestimate the prices of operating a lounge. If you just compare the food, sure...a couple of sandwiches and a coffee is not worth 20 euro. However keep in mind that:
- You can stay there for quite some hours if you'd like
- You will get free Wifi access
- You will get a proper spacious seat
- It will be quiet, the dragon will get you when your flight starts boarding
Sure the airline gets overcharged for using the lounge, but that happens because they can. But the fact of the matter is that you will receive personalized service throughout your stay and you wont have to sit on an uncomfortably small waiting pen seat. Space at an airport is expensive, and I assume is a rather big part of the entrance fee.
Like you said though, it depends on what you prefer. Some people prefer to spend that 20 euro at a bar and order a large bowl of spaghetti, others prefer to spend it on a more quiet environment and use the time for work. Its not called 'Business' lounge for nothing I guess ;-)