To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Starting price: 40 euros.
2. Average hammer price in the past: 43.5 euros.
3. Current condition: 30 (BC+), indicating acceptable but not excellent preservation.
4. Auction history: The coin has been auctioned 2 times and sold on both occasions, suggesting a consistent interest in the coin.

Given that the average hammer price in the past is 43.5 euros and the current starting price is 40 euros, we can infer that interest in the coin remains. However, the condition of 30 may limit the maximum price collectors are willing to pay.

Taking these factors into account, an estimated maximum purchase price could be slightly above the previous average hammer price, but considering the condition, it may be wise not to exceed too much.

Estimated maximum purchase price: 50 euros.

This price reflects an interest in the coin, but also takes into account its condition and price history.