Kinder Eggs aren't a German invention. I know....Germans will argue about this, but the idea really started in Italy under the Aprile....a small Italian company, which later got bought and integrated into Ferrero. It's basically cheap chocolate made into an egg, with a 'prize' in the middle which rests in a plastic shell.
Today, you can buy the egg for roughly 89-euro cents (roughly a dollar).
The prize is typically what kids and adults go after. I'd take a guess that there now at least fifty thousand prizes which have been developed and sold within the shell over the past forty years.
Some of the prizes have been collector items. Most have not.
My first real experience with the Kinder Eggs was in the 1994-timeframe when I was the office snacko for my Air Force shop. I was buying a number of things via the commissary, BX and off-base....and one day came to some display at a German shop where I could buy a stack of 100 Kinder Eggs for a heavily discounted price. I bought two stacks (200 eggs), and carried them back to the office. I figured....in three months....I'd sell all of them. Well....in my customer area of forty potential people....I sold out the whole thing in two weeks.
Later, I started to buy more at the commissary.....doing so until around 1998 when the commissary decided that they were too dangerous to sell (kids might swallow the toys). Yeah, it was rather bogus, but that's the mentality of dealing with Americans. Luckily, we don't consider beer, or Jack Daniels equally risky and avoid selling either.
I've worked with people who had 'toy' collections from the Kinder Eggs into the hundreds. Personally, I never got much into the deal. The chocolate is the cheap stuff (one step above Snickers) and the toy is some Chinese-made item which I don't see any big deal about putting together or collecting.
So, if you are an American and happen to be on your first big trip to Germany.....step into a regular shop and buy one Kinder Egg to experience the gimmick.