To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data:

1. Starting price: 200 euros.
2. Average hammer price in previous auctions: 205.94 euros.
3. Hammer price history: Hammer prices have varied, with some reaching up to 340 euros in previous auctions.
4. Condition: There is no specific information about the condition of this coin, which may make its value more uncertain.

Given that the average hammer price is approximately 205.94 euros and considering that higher hammer prices have been reached in the past (up to 340 euros), we can make an estimation.

If we take into account the starting price of 200 euros and the average hammer price, a reasonable maximum purchase price could be in the range of 250 to 300 euros, as this aligns with previous hammer prices and the starting price.

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