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

1. Current Condition: The coin has a condition of 75 (MBC), which indicates a fairly good state. In the past, coins with a similar condition have reached auction prices that vary, but are generally in a higher range than the starting price.

2. Price History: By observing previous auction prices for coins with a condition of 75, we can see that they have reached auction prices of up to 55 euros in recent auctions. However, there are also examples of coins with similar conditions that have sold for less, such as 15 euros.

3. Average Auction Price: The average auction price for this coin is 31.38 euros, suggesting that, in general, buyers are willing to pay more than the starting price.

4. Recent Trends: In the most recent auctions, the auction price has shown a tendency to be higher, especially for coins in good condition. For example, in a recent auction, a similar coin was sold for 50 euros.

5. Starting Price: The starting price is 18 euros, which is relatively low compared to previous auction prices.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 40 and 55 euros, considering that the market may be willing to pay a bit more for a coin in good condition and with a favorable price history.

Therefore, a reasonable maximum purchase price could be 50 euros.