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. Auction History:
- The average hammer price for this coin is 137.71 euros.
- In previous auctions, the hammer price has varied, but there are examples of sales exceeding 200 euros, such as the sale of 236 euros in October 2020.
- The last auction in which this coin was sold (2023-12-13) had a hammer price of 260 euros, suggesting significant interest in this coin.

3. Starting Price and Average Starting Price:
- The current starting price is 90 euros, which is above the average starting price of 71.03 euros. This may indicate that the seller has higher expectations due to demand or the coin's condition.

4. Price Trends:
- Observing the hammer prices in previous auctions, it seems there is potential for the hammer price to exceed 200 euros, especially considering that the condition is good and there is a history of high prices.

Based on these factors, an estimated maximum purchase price could be in the range of 200 to 250 euros. This is based on the demonstrated interest in previous auctions and the good condition of the coin. However, it is important to note that the final price may vary depending on competition in the auction and other market factors at the time of sale.