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

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

2. Historical Prices: By looking at the auction prices of coins with a similar condition (30) in the past, we can see that the auction prices have varied. For example, in previous auctions, coins with a condition of 30 have had hammer prices ranging from 15 to 25 euros.

3. Average Hammer Price: The average hammer price for this coin is 75.57 euros, but this includes coins in better condition. Given that the condition of this coin is lower, the actual hammer price is likely to be lower.

4. Starting Price: The starting price is 12 euros, which suggests that the auctioneer expects moderate interest in the coin.

5. Sales Trends: The coin has been auctioned 205 times, of which it has sold 174 times. This indicates a good sales rate, which may influence demand and, therefore, the price.

6. Prices of Similar Coins: In recent auctions, coins with similar conditions have had hammer prices ranging from 15 to 30 euros.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 20 and 30 euros. This reflects a balance between the current condition, historical prices, and market demand.