Based on the provided data, we can see that the coin in question has a starting price of 50 euros in the upcoming auction. It has been auctioned 6 times in the past, with an average closing price of 103.67 euros. The conservation of this coin is described as 25 (BC).

Additionally, we are given previous auction prices from past auctions:

1. Auction on 2020-07-02: Starting price 75, Estimated price 125, Closing price 76, Conservation 80.
2. Auction on 2021-03-16: Estimated price 20, Closing price 21, Conservation 25.
3. Auction on 2022-01-25: Estimated price 50, Closing price 260, Conservation 75.
4. Auction on 2022-01-25: Estimated price 220, Closing price 150, Conservation 70.
5. Auction on 2022-04-28: Starting price 25, Estimated price 40, Closing price 65, Conservation 70.

Given that the coin is in a conservation state of 25 (BC) and in the last auction had a starting price of 25 euros and an estimated price of 40 euros, we can estimate a maximum purchase price based on this data.

Considering the upward trend in closing prices, as well as the conservation of the coin, we could estimate a maximum purchase price around 60-70 euros. It is important to note that this is just an estimation and the final price may vary depending on other factors in the auction.