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

1. Condition of the coin: The coin has a condition rating 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. Current price and previous prices: The current starting price is 450 euros. In previous auctions, coins with similar conditions have had hammer prices that vary, but many have exceeded 300 euros. For example, in recent auctions, coins with a condition of 85 have reached hammer prices of up to 440 euros.

3. Price trends: Observing the hammer prices of coins with conditions of 85 and 90, it seems there is a trend for hammer prices to be between 400 and 500 euros for coins in good condition.

4. Average hammer price: The average hammer price for similar coins is 330.76 euros, but since the condition of this coin is superior, we can expect the hammer price to be higher.

5. Analysis of recent auctions: In the most recent auctions, hammer prices for coins with conditions of 90 have been in the range of 400 to 500 euros.

Based on these factors, an estimated maximum purchase price for this coin could be between 480 and 500 euros, considering that collector interest and the quality of the coin may influence the final price.