Based on the provided data, we can observe that the coin has had a starting price in previous auctions of 60 euros, with an estimated price of 100 euros. The final auction price has ranged between 60 and 85 euros. The coin's condition is described as "BC+".

Given that the coin will be auctioned with a starting price of 30 euros and considering the historical information, we could estimate a maximum purchase price around 90-100 euros. This is because the final auction price has reached up to 85 euros in the past, and considering that the coin's condition is relatively good (BC+), it is reasonable to expect that the final auction price this time could exceed the starting price and approach the previous price range.

Therefore, an estimated maximum purchase price for this coin in the upcoming auction could be around 90-100 euros.