Based on the data provided, we can observe that the coin in question has had an average auction closing price of 16.81 euros in previous auctions. Additionally, its conservation grade is 100 (UNC), indicating that it is in excellent condition.

Considering that the starting price for the next auction is 10 euros and the historical average closing price has been 16.81 euros, we could estimate a maximum purchase price around that value or slightly higher, taking into account the upward trend in closing prices.

Therefore, an estimated maximum purchase price for this coin in the next auction could be around 17-18 euros. It is important to note that this is just an estimate and the final price will depend on the demand at the auction and other factors that may influence the bidding.