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

1. Current Condition: The coin is in a condition of 90 (EF), which indicates that it is in quite good shape. This generally translates to a higher value compared to coins in lower conditions.

2. Auction History:
- The average hammer price for similar coins has been 68.17 euros.
- In previous auctions, hammer prices have varied significantly, but there are examples of sales in the range of 60 to 140 euros for coins with similar conditions (for instance, 75 euros on several occasions for coins in conditions of 90 or 95).

3. Starting Price and Average Starting Price:
- The current starting price is 45 euros, which is below the average starting price of 48.76 euros. This may indicate that there is a buying opportunity at a relatively low price.

4. Sales Trends: The coin has been sold in 19 out of the last 23 auctions, suggesting that there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 80 to 100 euros. This is based on the auction price history for coins in similar conditions and the demonstrated interest in previous auctions.

Therefore, a reasonable maximum purchase price could be 90 euros.