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

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

2. Auction History:
- The average hammer price for similar coins in the past is 84.35 euros.
- The coin has been auctioned 26 times, of which it has sold 20 times, indicating good demand.
- Previous hammer prices for coins with similar conditions (95) have reached up to 520 euros, although this may be an exceptional case.

3. Starting Price and Average Starting Price:
- The current starting price is 30 euros, which is quite low compared to the average starting price of 53.77 euros.

4. Price Trends:
- In previous auctions, hammer prices have varied, but there are examples where coins with similar conditions have reached hammer prices of 90 euros or more.

Based on these factors, an estimated maximum purchase price could be in the range of 80 to 120 euros, considering that the condition is good and that the price history suggests it could reach a higher value at auction.

Therefore, a reasonable estimated maximum purchase price could be 100 euros.