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

1. Coin Condition: The coin has a condition rating of 30 (BC+), which indicates that it is not in very good shape. This generally negatively affects its value compared to coins in better condition.

2. Historical Auction Data: By observing the hammer prices of coins with similar conditions, we can see that the hammer prices for coins in higher condition grades (70 and 80) have reached prices between 20 and 55 euros. However, for coins in lower conditions, like the one we are evaluating, the hammer prices tend to be lower.

3. Average Hammer Price: The average hammer price for this coin in the past has been 153.86 euros, but this includes coins in better conditions. For a coin with a condition rating of 30, it is reasonable to expect a significantly lower hammer price.

4. Starting Price and Previous Prices: The current starting price is 12 euros, and in previous auctions, coins with similar conditions have had hammer prices ranging between 20 and 45 euros.

Taking all these factors into account, an estimated maximum purchase price for this coin, considering its condition and historical prices, could be in the range of 25 to 35 euros. This reflects a price that is reasonable given its state and the market context.