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

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

2. Auction History:
- The coin has been auctioned 11 times, of which it has sold 9 times. This suggests that there is consistent interest in the coin.
- Previous auction prices for coins with a similar condition (30) have been 80 euros (2019), 33 euros (2022), and 25 euros (2019). This gives us a range of auction prices to consider.

3. Average Auction Price: The average auction price for this coin is 147.67 euros, but this includes coins in better condition. Given that the current condition is 30, it is likely that the auction price will be lower than this average.

4. Starting Price: The current starting price is 18 euros, which is quite low compared to previous auction prices.

5. Price Trends: Looking at previous auction prices, it seems that the range for a coin in condition 30 could be between 25 and 80 euros, with a more common price around 30-40 euros.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 40-50 euros, considering that interest in the coin may lead to a price slightly higher than the lowest auction price in the past.