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

1. 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 in previous auctions is 54 euros.
- The coin has been auctioned 13 times, of which it has sold 10 times, indicating good demand.
- Previous hammer prices varied, but there are examples of hammer prices reaching up to 96 euros in similar conditions.

3. Starting Price and Estimated Prices:
- The current starting price is 20 euros, which is relatively low compared to the average hammer price.
- The average starting price in previous auctions is 60.64 euros, suggesting that the current starting price may be underestimated.

4. Price Trends:
- In the most recent auction, the estimated price was 90 euros, and the hammer price was 85 euros, indicating potential for price increase.

Considering all these factors, an estimated maximum purchase price for this coin could be in the range of 80 to 100 euros, taking into account its good condition and the price history in previous auctions. A price of 90 euros could be a reasonable target, given that it has been reached in recent auctions.