To estimate a maximum purchase price for the coin in question, let’s consider the following factors:

1. Starting Price: 90 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 20 (BC-), indicating that it is in quite poor condition. This generally negatively affects the price, as coins in better condition tend to fetch higher prices.
3. Auction History: The coin has been auctioned once and sold at that time for the same starting price (90 euros). This suggests that the market has valued this coin within that range, although the low condition may limit collectors' interest.

Given that the condition is low and the previous hammer price was equal to the starting price, it is reasonable to assume that interest in this coin will not be very high.

Taking these factors into account, an estimated maximum purchase price could be around 90 euros, as it is the starting price and reflects the value that the market has previously assigned. However, since the condition is low, the actual hammer price may be lower than this, especially if there are other more attractive lots in the auction.

Therefore, an estimated maximum purchase price could be 90 euros, but with the caveat that it could be lower depending on the competition in the auction and the interest of collectors.