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

1. Current Condition: The coin has a condition of 90 (EF), which indicates 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 similar coins in the past is 348.13 euros.
- The last auction of a coin with a similar condition (90) was sold for 550 euros (auction date: 2018-10-17).
- In recent auctions, hammer prices have varied, but there are examples of coins with conditions of 85 and 95 that sold for 320 and 1600 euros, respectively.

3. Starting Price: The starting price of 500 euros is a good indicator of the seller's expectations, but it does not necessarily reflect the final hammer price.

4. Market Trends: Given that the coin has been auctioned 47 times and has had a high sales percentage (44 out of 47), this suggests there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 800 euros, considering that the condition is high and there are precedents for successful sales in that range. However, given the starting price of 500 euros, it is possible that the final hammer price may not significantly exceed this value unless there is active competition among bidders.

Therefore, a reasonable maximum purchase price could be 700 euros.