Based on the provided data, we can see that the coin in question has a conservation grade of 30 (BC+), with a starting price in the last auction of 12 euros and an estimated price of 20 euros. The final auction price in the last auction was 20 euros.

Considering that the average auction price for this coin is 27.5 euros and it has been auctioned twice in the past with an average starting price of 12 euros, we can estimate a maximum purchase price for this coin in the next auction.

Given that the estimated price in the last auction was 20 euros and the average auction price is 27.5 euros, we could consider a maximum purchase price around 30 euros, taking into account the coin's conservation and auction history.

Therefore, an estimated maximum purchase price for this coin in the next auction could be around 30 euros.