To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the general trend of the auctions.

1. Current starting price: 300 euros.
2. Average hammer price in the past: 316.8 euros.
3. Previous hammer prices:
- In several auctions, hammer prices have ranged from 250 to 420 euros, with a notable hammer price of 500 euros in a recent auction.
4. Condition: There is no specific information about the condition of this coin, which may affect its value. However, there have been previous auctions with conditions of 95 and 100 that have reached high hammer prices.

Given that the average hammer price is 316.8 euros and considering that recent auctions have reached hammer prices of up to 500 euros, an estimated maximum purchase price could be set in a range above the average hammer price, but not as high as the historical maximum, especially since there is no information on the condition.

Estimated maximum purchase price: 400 euros.

This price reflects a balance between the average hammer price and the higher prices reached in previous auctions, taking into account the lack of information about the current condition of the coin.