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 price compared to coins in lower conditions.

2. Current Starting Price and Previous Prices: The current starting price is 170 euros, and the average hammer price in previous auctions is 266.59 euros. This suggests that the coin has a market value that could be considerably higher than the starting price.

3. Auction History: In the past, the coin has had hammer prices that varied significantly, from 25 euros to 1400 euros, depending on the condition and demand at that time. However, the higher prices have been seen in coins with similar or better conditions.

4. Recent Trends: Looking at the most recent auctions, there is an increase in hammer prices, which could indicate a rising trend in interest for this coin.

5. Price Estimates: Based on the average hammer price of 266.59 euros and considering that the condition of the coin is good, an estimated maximum purchase price could be in the range of 300 to 400 euros, depending on the competition in the auction and the interest from other bidders.

Therefore, an estimated maximum purchase price for this coin could be 350 euros. This allows for a margin to compete in the auction, taking into account the price history and current condition.