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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in relatively good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for similar coins in the past is 624.47 euros.
- The coin has been auctioned 22 times, of which it has sold 20 times, indicating good demand.
- In previous auctions, hammer prices have varied, but there are examples of sales reaching up to 1000 euros for higher conditions.

3. Starting and Hammer Prices:
- The current starting price is 400 euros, which is a reasonable starting point.
- In previous auctions, the hammer price has exceeded the starting price on several occasions, suggesting that buyers are willing to pay more.

4. Recent Trends:
- In the most recent auction, the hammer price was 1000 euros for a condition of 85, indicating that there is potential for an increase in the hammer price for coins in good condition.

Based on these factors, an estimated maximum purchase price could be in the range of 700 to 800 euros. This is based on the hammer price history for coins with similar conditions and the trend of prices in recent auctions.

Therefore, a reasonable maximum purchase price could be 800 euros.