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

1. Current condition: The coin has a condition of 75 (MBC), indicating that it is in quite good shape.

2. Auction history:
- In the 2020 auction, a coin with a condition of 70 sold for 30 euros, with a starting price of 15 euros.
- In 2017, although there is no information on the condition, it sold for 20 euros with a starting price of 10 euros.
- In 2022, the same condition of 75 sold for 36 euros, with a starting price of 30 euros.

3. Starting and final prices:
- The current starting price is 30 euros, and the average final price in the past has been 28.67 euros.
- The average starting price in previous auctions has been 21.25 euros.

4. Trends: The trend shows that as the condition improves, the final price also tends to increase. The last auction with a condition of 75 reached a final price of 36 euros.

Based on this data, an estimated maximum purchase price could be in the range of 36 to 40 euros, considering that the condition is good and that interest in the coin may have increased. Therefore, a reasonable maximum purchase price could be 40 euros.