Based on the provided data, we can observe that the coin has had an average auction price of 69.64 euros in the past. Additionally, the condition of this coin has been described as "30 (BC+)".

Considering that the condition is relatively low and the average auction price has been 69.64 euros, we could estimate a maximum purchase price around 70-75 euros. Given that the starting price in this auction is 25 euros, it is likely that the final auction price will be in a range close to this estimate.

Therefore, an estimated maximum purchase price for this coin in the upcoming auction could be around 70-75 euros.