Based on the provided data, we can observe that the coin in question had a starting price of 12 euros in the current auction, with a conservation grade of 30 (BC+). Additionally, it has been auctioned five times in the past, being sold in three of them.

The average starting price of this coin in previous auctions has been 19.2 euros, while the average final price has been 28 euros. Regarding the previous prices of this coin, the following have been recorded:

- In the auction of December 22, 2020, the coin started at a price of 25 euros.
- In the auction of December 14, 2016, the coin started at a price of 9 euros and was estimated at 15 euros, being sold for 9 euros.
- In the auction of April 28, 2022, the coin started at a price of 25 euros and was estimated at 40 euros, being sold for 40 euros.
- In the auction of May 12, 2023, the coin started at a price of 25 euros and was estimated at 50 euros.

Given that the conservation grade of the coin is 30 (BC+), and considering the previous prices and the average final price, as well as the fact that it has been sold in three out of the five previous auctions, we could estimate a maximum purchase price for this coin at around 35-40 euros. It is important to note that this is just an estimate and the final price may vary depending on the demand in the current auction.