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

1. Current Condition: The coin has a condition of 30 (BC+), indicating that it is not in the best shape. This generally affects the auction price.

2. Auction History:
- The coin has been auctioned 9 times, of which it has sold 8 times. This indicates good demand.
- Previous auction prices varied, but there are some relevant prices to consider:
- In 2020, it sold for 18 euros with a condition of 70.
- In 2020, it sold for 65 euros with a condition of 80.
- In 2021, it sold for 25 euros with a condition of 80.
- In 2022, it sold for 20 euros with a condition of 70.
- In 2024, it sold for 35 euros with a condition of 80.

3. Starting Price and Average Price:
- The current starting price is 12 euros, and the average starting price in previous auctions is 32.78 euros, with an average hammer price of 41.38 euros.

4. Price Trends:
- Hammer prices have fluctuated, but it seems there is a price range that remains between 18 and 65 euros, depending on the condition.

Given that the current condition of the coin is 30 (BC+), which is lower than the conditions of the coins that have sold for higher prices, it is reasonable to adjust the expected hammer price downward.

### Maximum Purchase Price Estimate:
Considering all these factors, a reasonable maximum purchase price could be in the range of 20 to 30 euros. This is based on the fact that the condition is not high, but there is a sales history suggesting it could reach a higher price than the current starting price.

Therefore, an estimated maximum purchase price could be 25 euros.