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 reduces its value compared to coins in better condition.

2. Historical Prices: By looking at the auction prices of coins with similar conditions, we can see that the auction prices for coins with a condition of 30 have varied, but they are generally found in a lower range than coins in better condition.

3. Starting Price and Average Hammer Price: The current starting price is 12 euros, and the average hammer price for this coin in the past has been 128.83 euros. However, given that the condition is low, it is unlikely to reach that average price.

4. Recent Trends: In recent auctions, the hammer prices for coins with similar conditions have been in a range of 15 to 36 euros.

Based on these factors, an estimated maximum purchase price for this coin, considering its condition and historical prices, could be between 20 and 30 euros. This reflects a balance between the value of the coin in its current state and the interest of collectors.