To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current condition of the coin.

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

2. Previous Auction Prices: Looking at the auction prices of coins with similar conditions:
- In 2020, a coin with a condition of 30 was auctioned for 12 euros.
- In 2022, a coin with a condition of 70 was auctioned for 37 euros.
- In 2023, a coin with a condition of 30 was auctioned for 40 euros.

3. Average Auction Price: The average auction price for this coin is 66.15 euros, but this includes coins in better condition.

4. Auction Trends: The coin has been auctioned 16 times, of which 14 have sold. This indicates a consistent interest in the coin, which may influence its price.

5. Starting Price: The starting price is 20 euros, suggesting that the seller has moderate expectations.

Based on these factors, an estimated maximum purchase price for the coin, considering its condition and previous auction prices, could be between 30 and 40 euros. This aligns with previous auction prices for coins of similar condition and the interest in the auction.

Therefore, an estimated maximum purchase price could be 40 euros.