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

1. Condition: The coin has a condition rating of 30 (BC+), indicating that it is not in very good shape. This will affect its value compared to coins in better condition.

2. Previous Prices:
- In previous auctions, we have seen hammer prices that vary significantly. For example, a coin with a similar condition (30) sold for 18 euros and another for 15 euros. However, there is a hammer price of 160 euros for a coin with a condition of 85, indicating that the value can increase significantly with better condition.
- The average hammer price for similar coins is 75.75 euros, which can serve as a reference.

3. Starting Price and Estimates: The current starting price is 12 euros, which is quite low compared to the average hammer price. This suggests that there could be interest in the coin, especially if the starting price is attractive.

4. Sales Trends: The coin has been auctioned 5 times, of which it has sold 4 times. This indicates that there is an active market for this coin, which may increase its value.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 30 to 50 euros. This is based on the current condition and previous hammer prices for coins in similar conditions. A price of 50 euros could be a reasonable limit, considering that interest in the auction could lead to a higher bid.