Based on the data provided, we can observe that the coin in question has had an average auction closing price of 283.62 euros in previous auctions. Additionally, the current condition of the coin is graded at 85 (VF+/XF-).

Considering that the starting price in the upcoming auction is 400 euros and the historical average closing price is 283.62 euros, we can estimate a maximum purchase price based on this data.

Given that the condition of the coin is relatively good (85), and considering that in previous auctions it has achieved prices higher than the current starting price, we could consider an estimated maximum purchase price around 350-380 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 pay attention to the auction to adjust the offer accordingly.