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 (EF), indicating 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 600 euros. In the past, we have seen auction prices that varied significantly, but there are examples of similar coins in comparable condition that have sold for significantly higher prices. For instance, in 2019, a coin with a condition rating of 85 sold for 450 euros, and in 2018, another with the same condition sold for 600 euros.

3. Price trends: The average auction price for similar coins is 78.63 euros, but this may not reflect the true value of a coin in EF condition. Given that the coin in question has a higher condition, it is reasonable to expect its auction price to be above this average.

4. Supply and demand: The coin has been auctioned 100 times in the past, with an 82% sell-through rate, indicating good demand. This suggests that buyers are willing to pay higher prices.

5. Comparison with previous prices: By looking at the auction prices of coins with similar conditions, we can see that prices have fluctuated, but there are examples of sales above 600 euros.

Taking all these factors into account, an estimated maximum purchase price for this coin could be between 800 and 1000 euros, considering its good condition and the price trends in the market. However, the final price will depend on the competition in the auction and the willingness of other buyers to pay.