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

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

2. Auction History: By looking at previous hammer prices for coins with similar conditions (90 and 95), we can see that the hammer prices have varied. For example, in recent auctions, coins with a condition of 90 have reached hammer prices ranging from 12 to 35 euros.

3. Average Hammer Price: The average hammer price for this coin is 27.36 euros, which gives us a good reference point.

4. Recent Trends: In the most recent auctions, hammer prices for coins with a condition of 90 have been in the range of 20 to 35 euros.

5. Starting Price and Estimated Prices: The starting price is 12 euros, and the average starting price is 16.14 euros. However, given that the condition is good, it is reasonable to expect that the hammer price will be above these values.

Based on these factors, an estimated maximum purchase price for the coin could be in the range of 35 to 45 euros. This is based on the historical hammer prices for coins of similar condition and the potential interest in the auction.