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

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

2. Auction History:
- The average hammer price for similar coins in the past is 122.88 euros.
- However, the hammer prices for coins with similar conditions (30) are lower. For example, at the 2021 auction, a coin with a condition of 30 was sold for 14 euros.

3. Starting and Hammer Prices:
- The current starting price is 12 euros, which is relatively low.
- In previous auctions, starting prices have varied, but the highest hammer price for similar conditions has been 45 euros.

4. Sales Trends: The coin has been auctioned 10 times, of which it has sold 8 times, indicating good demand, although the hammer price can vary significantly.

5. Estimation Based on Previous Data: Given that the condition is low and considering the price history, a reasonable hammer price for this coin could be in the range of 15 to 30 euros, based on the lowest hammer price for similar conditions.

Conclusion: Taking all these factors into account, an estimated maximum purchase price for this coin could be approximately 30 euros. This would allow for competitive bidding in the auction without exceeding a reasonable limit given the coin's condition.