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

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

2. Previous Auction Prices: By looking at previous auction prices, we can see that the coin has reached auction prices of up to 1000 euros in superior conditions (such as in 2018 with a condition rating of 100) and has had auctions in the range of 170 to 400 euros in conditions similar to the current one (condition rating of 85-90).

3. Average Auction Price: The average auction price for this coin is 204.53 euros, providing a good reference for the expected value.

4. Recent Trends: In the most recent auctions, the hammer prices have varied, but there are examples of auctions exceeding 300 euros in similar conditions.

5. Starting Price: The starting price is 100 euros, indicating that the seller expects a reasonable minimum price.

Based on these factors, an estimated maximum purchase price could be in the range of 300 to 400 euros, considering that the condition is good and there are precedents for auctions in that range. However, given the price history and trend, a reasonable maximum purchase price could be 350 euros.