To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 12 euros.
2. Current grade: 70 (MS-), indicating good preservation.
3. Auction history:
- In 2016, the coin was auctioned with a starting price of 70 euros and sold for 70 euros, although there is no information on the preservation.
- In 2018, it was auctioned with a starting price of 10 euros and sold for 12 euros, with a grade of 30, which is quite low.
- In 2022, it was auctioned with a starting price of 12 euros and sold for 20 euros, with a grade of 70, similar to the current one.

4. Average hammer price: 33 euros, indicating that, in general, coins of this reference tend to sell for higher prices than the starting price.

Given that the current grade is good (70) and considering the average hammer price of 33 euros, as well as the hammer price of 20 euros in the most recent auction with the same grade, we can estimate a maximum purchase price.

Taking all these factors into account, a reasonable maximum purchase price could be in the range of 25 to 30 euros, as it reflects the good preservation and the auction history.

Therefore, the estimated maximum purchase price would be 30 euros.