Based on the data provided, we can observe that the coin in question has had an average auction closing price of 277.56 euros in previous auctions. Additionally, its current condition is estimated at 70 (MBC-).

Considering that the coin's condition is relatively good (70), and that the average auction closing price in previous auctions has been 277.56 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 upcoming auction could be around 280-300 euros, considering its condition and the history of previous closing prices.