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

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

2. Auction History:
- The average hammer price for similar coins in condition 90 has been between 18 and 34 euros in previous auctions.
- In past auctions, coins with condition 90 have sold for prices ranging from 18 to 34 euros, with a starting price of 18 euros in a recent auction.

3. Average Starting and Hammer Price:
- The average starting price is 31.16 euros and the average hammer price is 33.86 euros. This suggests that the market is willing to pay close to these values.

4. Price Trends:
- In previous auctions, the hammer price has varied, but coins in similar conditions have reached prices of up to 90 euros in the past, although this may be an exceptional case.

Based on this data, an estimated maximum purchase price for the coin in condition 90 could be in the range of 35 to 50 euros, considering that the average hammer price is 33.86 euros and that the condition is good. However, since the starting price is 15 euros, it is likely that the final price will not significantly exceed this range.

Therefore, a reasonable estimated maximum purchase price could be 45 euros.