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 (EF), indicating that it is in good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices, we can see that the coin has reached auction prices of up to 3700 euros in the past, although it has also had lower auction results. The average auction price in previous sales is 2294.44 euros.

3. Starting Price and Average Starting Price: The current starting price is 1900 euros, which is below the average starting price of 1963.64 euros. This could indicate that there is a buying opportunity at a reasonable price.

4. Sales Trends: The coin has been sold in 10 out of 12 auctions, suggesting that there is consistent interest in it.

5. Comparison with Similar Condition Prices: In previous auctions, coins with similar conditions (85-90) have had auction prices ranging from 1500 to 3700 euros.

Taking all these factors into account, an estimated maximum purchase price could be around 2500-2700 euros. This range considers the good condition of the coin and previous auction prices, as well as the demonstrated interest in past auctions.