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

1. Current Condition: The coin has a condition 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. Historical Prices: By looking at previous auction prices for coins with similar conditions (in this case, 90 and 95), we can see that auction prices have fluctuated. For example, in recent auctions, coins with a condition of 95 have sold for around 410 to 670 euros. However, for condition 90, the auction prices have been closer to 320 to 400 euros.

3. Starting Price and Average Auction Price: The current starting price is 450 euros, which is higher than the average auction price of 320.09 euros. This suggests that the seller has high expectations, but it may also indicate that the market is willing to pay more for this particular coin.

4. Market Trends: Since the coin has been auctioned 36 times and has had a sell-through rate of 97.2% (35 out of 36), this indicates strong demand. Additionally, the average starting price has been increasing, which may reflect growing interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 500 euros, considering that the condition is good and there is a price history that supports this estimate. However, given the starting price of 450 euros, it might be reasonable to offer a little less, perhaps around 450 euros, to avoid exceeding the starting price.

Therefore, an estimated maximum purchase price could be 450 euros.