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

1. Condition: The coin to be auctioned has a condition rating of 80 (MBC+), which indicates that it is in good condition. This generally increases its value compared to coins with lower conditions.

2. Previous Prices: Previous auction prices are important for establishing a price range. In past auctions, the hammer prices were:
- 110 euros (condition 70)
- 40 euros (condition 30)
- 12 euros (condition 25)

The highest hammer price was 110 euros, suggesting that there is a market willing to pay that amount for coins in good condition.

3. Starting Price and Average Hammer Price: The current starting price is 90 euros, and the average hammer price in previous auctions was 54 euros. This indicates that the starting price is relatively high compared to the previous average hammer price.

4. Market Trends: Since the coin has sold in all previous occasions and there have been no failed attempts at sale, this suggests that there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 110 to 130 euros, considering that the condition is better than that of the coin that sold for 110 euros and that the starting price is 90 euros. However, since the starting price is relatively high, a more conservative purchase price could be around 110 euros, which is the highest recorded hammer price for a coin in similar condition.

Therefore, an estimated maximum purchase price could be 110 euros.