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+). This indicates that it is not in the best condition, which will affect its value.

2. Auction History: By observing the hammer prices of coins with similar conditions in the past, we can make a comparison. For example:
- In previous auctions, coins with conditions of 30 have had hammer prices that varied, but are generally in the range of 15 to 25 euros.
- A coin with a condition of 25 was sold for 14 euros, and another with a condition of 30 was sold for 20 euros.

3. Starting Price and Average Price: The current starting price is 12 euros, and the average hammer price in the past has been 67.01 euros, but this includes coins in better conditions.

4. Recent Trends: In the most recent auctions, hammer prices for coins of similar condition have been around 15 to 25 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 20 and 25 euros, considering that the condition is not optimal and that the price history shows that coins in similar conditions typically do not exceed this range.

Therefore, a reasonable maximum purchase price could be 25 euros.