To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially considering the current grade of the coin (30 BC+) and the auction prices of coins with similar grades in the past.

1. Current Grade: The coin has a grade of 30 (BC+), indicating that it is not in the best condition. This generally translates to a lower auction price compared to coins in better condition.

2. Historical Auction Prices: By observing the auction prices of coins with similar grades (around 30), we find that:
- In previous auctions, coins with a grade of 30 have reached auction prices between 15 and 30 euros.
- For example, in the auction on 2018-12-11, a coin with a grade of 30 was sold for 15 euros.

3. Price Trends: The average auction price for this particular coin is 107.29 euros, but this includes coins in better condition. Given that the current grade is quite low, it is unlikely that this price will be reached.

4. Starting Price and Estimates: The starting price is 12 euros, and the average starting price in the past has been 54.55 euros. However, since the grade is low, it is reasonable to adjust expectations towards a lower auction price.

5. Final Estimate: Taking all these factors into account, a maximum estimated purchase price for this coin, considering its grade and historical auction prices, could be between 20 and 30 euros. This reflects a balance between the current grade and the auction prices of similar coins in the past.

Therefore, a maximum estimated purchase price for this coin could be 30 euros.