Based on the data provided, we can observe that the coin in question has had varying starting and final prices in previous auctions, as well as different price estimates. The conservation status of the coin has been described as 'BC-' (Bien Conservada menos).

Given that the current starting price is 9 euros and the average historical final price is 85.78 euros, we can see that there has been a significant increase in the final price compared to the starting price. Additionally, the conservation value of the coin has ranged between 70 and 90 in previous auctions.

Taking these data points into consideration, as well as the history of previous prices, we could estimate a maximum purchase price for this coin at around 100-120 euros, considering the 'BC-' conservation status and the upward trend in final prices. It is important to note that this is just an estimation and the final price could vary depending on factors such as demand at the auction and competition from other bidders.