Based on the data provided, we can observe that the coin has had an average auction closing price of 25.14 euros in previous auctions. Additionally, its current condition is 70 (VF+), indicating that it is in an acceptable condition.

Considering that the starting price this time is 15 euros and the historical average closing price has been 25.14 euros, we could estimate a maximum purchase price around that value or slightly higher, taking into account the coin's condition and auction history.

Therefore, an estimated maximum purchase price for this coin in the upcoming auction could be around 26 to 28 euros, considering the information provided. It is important to note that the final price will depend on the demand at the auction and other factors that may influence potential buyers' decisions.