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 quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price in previous auctions is 20.4 euros.
- The last auction with a similar condition (75) had a hammer price of 20 euros.
- In previous auctions, hammer prices have varied, but generally, coins with a condition of 70 have had hammer prices ranging from 12 to 18 euros, while those with a condition of 75 have reached up to 20 euros.

3. Starting Price and Estimated Price:
- The current starting price is 20 euros, and the estimated price is 30 euros. This suggests that there are expectations for the coin to sell for a price higher than the starting price.

4. Sales Trends: Of the 16 times it has been auctioned, 11 have resulted in sales, indicating a reasonable success rate.

Based on these factors, an estimated maximum purchase price could be around 25-30 euros. This is because, although the starting price is 20 euros, the condition of the coin and the price history suggest that it could reach a higher price at auction.

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