To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Current starting price: 600 euros.
2. Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape.
3. Auction history: It has been auctioned 2 times in the past, but it has not sold on either occasion, which may indicate that interest or demand has not been sufficient to reach the starting price.
4. Average starting price: 500 euros, which is lower than the current starting price.
5. Estimated price in the previous auction: In the June 2022 auction, the starting price was 400 euros and the estimated price was 700 euros, although no hammer price was recorded.

Given that the coin has not sold in previous auctions and considering the current starting price and the average starting price, it is reasonable to assume that interest in the coin may not be as high as expected.

Taking all these factors into account, an estimated maximum purchase price could be around 500-550 euros. This is based on the average starting price and the need to attract potential buyers, given that the current starting price is higher than the interest demonstrated in previous auctions.