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

1. Current starting price: 350 euros.
2. Coin condition: 70 (MBC-), indicating acceptable preservation.
3. Auction history:
- In 2020, the coin was auctioned with a starting price of 200 euros and sold for 340 euros.
- In 2016, the starting price was 225 euros, but the final price is not available.
- In 2022, the starting price was 300 euros and it sold for 305 euros.

4. Average prices:
- Average starting price: 268.75 euros.
- Average final price: 326.25 euros.

Given that the coin has a starting price of 350 euros, which is higher than both the average starting price and the average final price, and considering that it has previously sold for up to 340 euros, it is reasonable to assume that the final price could be in a similar range.

Taking into account the coin's condition and the price history, an estimated maximum purchase price could be between 340 and 360 euros, considering that collector interest may influence the final price.

Therefore, an estimated maximum purchase price could be 360 euros.