Based on the data provided, we can observe that the coin in question has had a good auction history, with an average auction price of 74.39 euros. Additionally, the coin's condition is graded at 90 (EBC), which is quite high.

Considering that the coin will be auctioned in the upcoming days with a starting price of 70 euros, we can estimate a maximum purchase price based on the average auction price and the coin's condition. Given the high condition and the average auction price of 74.39 euros, we could consider a maximum purchase price around 80-85 euros.

It is important to note that the final auction price may vary depending on the demand at that specific moment, so it is advisable to monitor the auction to adjust the offer accordingly.