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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: Looking at previous auction prices for coins with a similar condition (80), the most recent price was 75 euros in December 2020. There was also an auction for 40 euros in January 2023, suggesting that the price may fluctuate.

3. Average Auction Price: The average auction price for this coin is 76.3 euros, which provides a good reference for the expected value.

4. Starting Price: The current starting price is 40 euros, which is relatively low compared to previous auction prices.

5. Sales Trends: The coin has been auctioned 12 times, of which it has sold 10 times, indicating good demand.

Based on these factors, an estimated maximum purchase price could be around 75-80 euros, considering that the condition is good and the average auction price is in that range. However, since the market can be volatile, it is advisable not to exceed 80 euros to avoid overpaying.

Therefore, a reasonable maximum purchase price could be 80 euros.