I'm not an active user of this forum but it helped me a lot with this matter, so I've decided to share my experience here. Hopefully it will be a good advice for others too.
I regularly go (once a year) to my lifetime dentist (which is not in Switzerland nor in Germany). Last here he saw a very tiny caries (from an XRay) but he said that was nothing to worry about, and by just by keeping the same dental cleaning standards it wouldn't have given me any issue. Fine.
As this year I couldn't manage to 'visit' him, I decided to find a dentist nearby Zürich. I've always heard that Swiss dentists are charging scary fees, therefore I decided straightaway to find a dentist in Germany (Konstanz). I saw an advertisement on a tram and I fixed an appointment.
After he visits me, he tells me that from the XRay there is a caries and he explains me that I'll be required to go back twice, to complete its removal, and that he will fill the hole with ceramic (which is the only material that he would use for this case). He tells me that the bill will be around 600 euros.
As I trust my dentist back home, I decided that it was worth to go somewhere (this time in Zürich) to hear a second opinion.
I got an appointment for the same day in which I called (I love Zürich!!!). The whole process starts again, but this time with a different outcome.
After the check up and the XRays the Züri dentist tells me that he sees a tiny caries (always between the same teeth) which he starts removing immediately, just to see if there are other cavities in the tooth next to it. There is nothig else, he fills the hole, and he asks me to go back in 1 year. The whole process lasted around one hour and the bill (including the XRays) will be between 150-200 CHF.
Obviously this is totally in line with what my dentist told me last year, but what about the German dentist? Is he just trying to rip off clients (especially those coming from Switzerland)?
I hope my experience will be enlightening, and just to remember people that what is on the other side of the border is not always the cheapest option.