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 450 euros. In previous auctions, the average hammer price has been 354.02 euros, suggesting that coins of this reference tend to sell below their starting price.

3. Sales History: The coin has been auctioned 57 times, of which 51 have been successful sales. This indicates good demand and consistent interest in this coin.

4. Previous Hammer Prices: Looking at previous hammer prices for coins with similar conditions (80-90), we can see that they have reached hammer prices that vary, but some have exceeded 400 euros, especially in higher conditions.

5. Recent Trends: In the most recent auctions, hammer prices have shown a trend of stabilizing or slightly increasing, which could indicate a rise in demand.

Based on these factors, an estimated maximum purchase price could be in the range of 500 to 600 euros. This is because, although the starting price is 450 euros, the good condition and price history suggest that there could be competition in the auction, which could lead to a higher hammer price.

Therefore, a reasonable maximum purchase price could be 550 euros.