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

1. Current Condition: The coin has a condition rating of 85 (ebc-), indicating that it is in quite good shape. This is important because coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for similar coins in condition 85 has been approximately 641.67 euros.
- In previous auctions, we have seen hammer prices that varied, but some have reached up to 1300 euros in exceptional cases.

3. Starting Price and Average Starting Price:
- The current starting price is 500 euros, which is slightly lower than the average starting price of 516.25 euros. This could indicate a good buying opportunity.

4. Sales Trends: The coin has been auctioned 21 times, of which 15 have been sold. This suggests that there is consistent interest in this coin, which may influence its price.

5. Previous Prices:
- In previous auctions, the highest hammer price for a coin with a similar condition has been 950 euros, which can serve as a reference point.

Taking all these factors into account, an estimated maximum purchase price could be around 800-900 euros. This is based on the hammer price history for coins in similar conditions and the demonstrated interest in previous auctions. However, since the starting price is 500 euros, it could be a good opportunity to acquire it at a lower price if competition is not high.