Based on the provided data, the coin will be auctioned with a starting price of 25 euros and is in a conservation grade of 70 (MBC-). Additionally, in a previous auction in 2015, the coin had a starting price of 300 euros, an estimated price of 500 euros, and was sold for 600 euros.

Given that the conservation grade of the coin is 70 (MBC-), we can assume that its condition is not excellent but not poor either. Considering that in the previous auction the coin was sold for 600 euros, and taking into account that the starting price is 25 euros, we could estimate a maximum purchase price around 550-600 euros, depending on the demand in the current auction.

It is important to note that the final auction price will depend on various factors, such as collectors' demand at that specific moment, so it is advisable to pay attention to the auction to adjust the offer accordingly.