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. Previous Prices: By looking at previous auction prices for coins with similar conditions (85-90), we can see that auction prices have varied, but there are examples of auctions reaching up to 460 euros for condition 90.

3. Average Auction Price: The average auction price for this coin is 283.25 euros, which provides a baseline for the expected value.

4. Starting Price: The current starting price is 350 euros, suggesting that the seller has expectations of a higher auction price.

5. Auction Trends: The coin has been auctioned 22 times, with a high sales percentage (20 out of 22), indicating consistent demand.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 460 euros, considering that the condition is good and there are precedents for auctions in that range. Therefore, a reasonable maximum purchase price could be 450 euros.