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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price of similar coins in the past is 63.67 euros.
- Previous auctions show that coins with similar conditions (90) have reached hammer prices that vary, but some examples are:
- 120 euros (2022-03-17)
- 100 euros (2020-12-17)
- 90 euros (2021-06-22)
- The trend shows that coins in EF condition can reach hammer prices ranging between 90 and 120 euros.

3. Starting Price: The starting price is 50 euros, which is relatively low compared to previous hammer prices.

4. Supply and Demand: Since there are no auctions scheduled in the coming days and the coin has been auctioned 28 times, demand may be moderate, but the sales history suggests there is interest in this coin.

Based on these factors, an estimated maximum purchase price could be between 90 and 120 euros, considering that the condition is good and that the hammer price history supports this estimate.

Therefore, a reasonable maximum purchase price could be 110 euros.