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 coin has been auctioned 21 times, of which it has sold 17 times. This indicates good demand.
- The average hammer price is 49.76 euros, suggesting that, on average, buyers are willing to pay around this amount.
- In previous auctions, hammer prices for coins with similar conditions (80) have varied, but there are examples where prices have reached 95 euros (such as in the auction on 2022-10-17).

3. Starting Price and Previous Prices:
- The current starting price is 20 euros, which is relatively low compared to the average hammer price.
- In previous auctions, starting prices have varied, but the highest hammer price for a similar condition has been 100 euros.

4. Price Trends:
- Observing the most recent auctions, there is a trend for hammer prices for coins in good condition (like the current one) to be higher, especially if there is interest in the coin.

Based on these factors, an estimated maximum purchase price for this coin could be between 80 and 100 euros, considering its good condition and the hammer price history in previous auctions. A price of 90 euros could be a reasonable target, as it falls in the middle of this range and reflects the price trends in the market.