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

1. Current Condition: The coin has a condition rating of 80 (MBC+). This is an important factor, as coins in better condition tend to fetch higher prices.

2. Auction History:
- The average hammer price for similar coins in the past is 33.5 euros.
- The condition of the coins auctioned in the past varies, but there are examples of coins with similar conditions (80) that have sold for prices ranging from 15 to 25 euros.

3. Starting and Estimated Prices:
- The current starting price is 12 euros, and the average starting price in the past has been 23.43 euros.
- The estimated price for this auction is 20 euros, suggesting that there are expectations for the coin to sell for a reasonable price.

4. Sales Trends: Of the 27 times it has been auctioned in the past, it has sold 21 times, indicating a good sales rate.

5. Comparison with Previous Prices:
- In previous auctions, coins with similar conditions have reached hammer prices of up to 30 euros or more, depending on demand and condition.

Based on these factors, a maximum estimated purchase price for this coin could be between 25 and 35 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate.

Therefore, a reasonable maximum purchase price could be 30 euros.