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 75 (MBC), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- It has been auctioned 11 times, of which it has sold 9 times. This indicates good demand.
- The average hammer price in previous auctions is 19 euros, providing a benchmark.
- In past auctions, hammer prices have varied, with a maximum of 30 euros in an auction with a condition rating of 80.

3. Starting Price and Estimates:
- The current starting price is 20 euros, which is slightly higher than the average starting price of 14.33 euros.
- The estimated price of 25 euros in the current auction is reasonable, given that similar prices have been estimated in the past for comparable conditions.

4. Price Trends:
- In previous auctions, hammer prices have ranged from 12 to 30 euros, depending on the condition and demand at that time.

Taking all these factors into account, an estimated maximum purchase price for this coin could be around 25-30 euros, given that the condition is good and there is a price history that supports this estimate. However, it is important to consider that the final price may vary depending on competition in the auction and the interest of other buyers.