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. Previous auction prices: By looking at previous hammer prices for coins with a similar condition (90), we can see that the hammer prices have varied, but they have generally been in a range between 300 and 400 euros. For example, in past auctions, coins with a condition of 90 have reached hammer prices of 360, 330, and 280 euros.

3. Starting price and average hammer price: The current starting price is 450 euros, which is higher than the average hammer price of 326.93 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 101 times in the past and has had a sell-through rate of 90%, this indicates a consistent interest in the coin, which may influence the price.

5. Previous estimated prices: Estimated prices for similar coins have varied, but many have been in the range of 400 to 500 euros.

Based on these factors, an estimated maximum purchase price for this coin could be around 400-450 euros. This is based on the condition, previous hammer prices, and market interest. However, since the starting price is 450 euros, buyers may be willing to pay a little more if there is competition in the auction.