To estimate a maximum purchase price for the coin that is going to be auctioned, we can analyze the provided data, especially the previous hammer prices and the current starting price.

1. Current starting price: 200 euros.
2. Previous hammer prices:
- In past auctions, hammer prices have varied considerably, with some reaching up to 370 euros (in 2016) and others remaining at 5 euros.
- The average hammer price in previous auctions is approximately 110.79 euros.

3. Condition: There is no information about the condition of this coin, which may affect its value. However, by observing the hammer prices of coins with similar conditions (around 80-90), it can be inferred that the price could be in a mid-range.

4. Auction trends: The coin has been auctioned 112 times, of which 90 have been sales. This indicates a consistent interest in the coin, which may increase its value.

5. Comparison with previous prices:
- In previous auctions, starting prices have ranged from 5 to 300 euros, with hammer prices reaching 340 euros on some occasions.
- The price trend seems to be increasing, especially in recent auctions.

Based on this data, an estimated maximum purchase price could be in the range of 250 to 300 euros, considering the current starting price of 200 euros and the previous hammer prices. This also takes into account the interest in the coin and the possibility that buyers may be willing to pay more in a competitive auction.

Therefore, an estimated maximum purchase price could be approximately 300 euros.