Construction costs are currently skyrocketing

The neighbor paid 15,000 for his 10 kWp system and feeds into the grid in summer for 6 cents/kWh.

Should I calculate for you? Per kWp you get about 1000-1100 kWh per year depending on region and orientation. Self-consumption 3000 kWh at 40c = €1200 -> just through self-consumption the system has paid off after 13 years. Now the neighbor also gets feed-in tariff on 7000-8000 kWh, which makes another €450 per year. The system is then paid off after 10 years. What do you do with the excess hot water in summer? How much do you get paid for it?

Shall I do the math for you?
Per kWp you get about 1000-1100 kWh per year depending on the region and orientation. Self-consumption 3000 kWh at 40c = €1200 -> the system has paid off after 13 years just through self-consumption.
Now the neighbor also gets feed-in tariff on 7000-8000 kWh, that makes another €450 per year. The system then pays off after 10 years. What do you do with the excess hot water in summer? How much do you get paid for that?

Yes. But you are not calculating...
My solar thermal system for €1,000 saves me roughly 2,000 kWh over the entire summer, so at the current gas price €1,000.
So it pays off after one year.

roughly estimated, 2,000 kWh

Then please don’t estimate "roughly" but exactly. First, how do you arrive at 2,000 kWh yield from the roof, second, for which summer consumption do you want to save that. Or, you know what, save it. In the last 6 years I haven’t even reached half with my 5m², on average about 800 kWh. With 13ct/kWh for gas, I saved about 100 euros this year. How much that was in the previous years (at a kWh price < 3ct for gas) you can surely calculate yourself.

Then please calculate not "roughly" but exactly. First, how you come to 2000 kWh yield from the roof, second, for which summer consumption you want to save it. Or, you know what, spare yourself that. In the last 6 years, I didn’t even reach half with my 5m2, on average 800 kWh. With 13ct/kWh for gas, I saved about ~100 euros this year. How much that was in the previous years (with a kWh price < 3ct for gas) you can surely calculate yourself.

I’m asking my colleague. From April 2022 to October 2022 he had no burner because it was defective. With 5 people in the household... But I already suspect that he will contradict your figures... But I’m not mad about that. Everyone should do it as they think. And whoever has the money can gladly throw it away... I have 2 children, I cannot waste my coal... sorry. Because, that has to be said clearly sometimes... in this forum there are many forum members who, consciously or unconsciously, have no children... Without children, you can of course plan quite differently...

... So I paid just under 1,000 EUR for my 5m² solar thermal panels. ...

I would be interested (as a former ST owner) to know where on earth you can get 5m² of ST for just under 1K. Of course including installation, the necessary scaffolding, the required solar storage tank, the control system, the solar piping (really expensive). Basically all the little things that are included in the price of a photovoltaic system here on the forum. Really, I have a genuine interest in finding out. I thought I got mine about 15 years ago at a bargain price of around 4K including installation and the aforementioned "little things" since my brother-in-law installed them and had good contacts at Weishaupt... But maybe I was ripped off?

Yes. But you’re not calculating... My solar thermal system for 1,000 EUR roughly saves me 2,000 kWh over the entire summer, so at the current gas price it’s 1,000 EUR. So it pays for itself after one year.

Where can I find this “current” gas price? I find values between 13-18 ct/kWh…