To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current condition: The coin has a condition of 75 (MBC), indicating that it is in quite good shape.

2. Auction history:
- The average hammer price in previous auctions is 23.67 euros.
- The hammer prices in previous auctions for coins with similar conditions (70 and 80) have varied, but there are examples of hammer prices of 26 euros and 30 euros for condition 80, and 35 euros for a condition of 70.

3. Starting price: The starting price is 20 euros, which may be a good indicator of the expected sale.

4. Price trends: Observing previous hammer prices, it seems that coins with conditions from 70 to 80 tend to sell between 25 and 35 euros.

Based on this data, an estimated maximum purchase price could be around 30-35 euros, considering that the coin's condition is 75 and that there have been previous hammer prices in that range. This also takes into account the starting price and the history of hammer prices.

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