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 90 (EF), indicating 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 has been 43.77 euros.
- In previous auctions, coins with a similar condition (90) have had hammer prices that varied, but some examples are:
- 26 euros (2024-04-24)
- 35 euros (2020-02-12)
- 40 euros (2022-10-18)
- 60 euros (2021-06-22)
- The trend shows that the price can range between 25 and 60 euros, depending on demand and other factors.

3. Starting Price and Average Starting Price:
- The current starting price is 18 euros, which is relatively low compared to the average hammer price.
- The average starting price is 32.77 euros, suggesting that the hammer price could be significantly higher.

4. Current Demand: There are 3 auctions scheduled in the coming days, which may increase competition and, consequently, the hammer price.

Based on these factors, an estimated maximum purchase price could be between 50 and 70 euros, considering that the condition is good and that the price history shows that similar coins have reached higher prices in the past.

Therefore, a reasonable maximum purchase price could be 65 euros.