Depends what you want and its not too expensive. You can start with 150 Euro per month (with 1500 deductible per year, 60% for dentist) with base insurance and from there it goes up dependent what you want (e.g. private room in hospital, low deductible etc.) to around 700 Euro. This is for private insurance, if you go with the state insurance it depends on your salary but the maximum is also around 700+ Euro. There a non working spouse and children would be included while in a private insurance it is not.

With his income I doubt health insurance is a big deal for him in Germany. Taxes is a different story. Also you have a really hard time to buy ETF's or other investments (basically you end up in a twilight zone because of US and EU regulations each locking you out of the others investments).