To estimate a maximum purchase price for the coin in question, we can analyze the provided data. Here are some key points to consider:

1. Starting price: 100 euros.
2. Average hammer price: 107.43 euros, indicating that, on average, coins of this reference sell for above their starting price.
3. Auction history: The coin has been auctioned 29 times and has sold on all those occasions, suggesting consistent demand.
4. Previous hammer prices: Hammer prices vary, but there are examples of auctions reaching up to 500 euros in high conservation conditions (conservation value 100).

Since there is no information about the conservation of the current coin, it is prudent to consider a price range based on the auction history. By observing the highest hammer prices and the average hammer price, we can make an estimation.

- Highest hammer prices: 500 euros (in conservation condition 100).
- Lowest hammer prices: 10 euros (in very low conservation condition).

Given that the starting price is 100 euros and the average hammer price is 107.43 euros, an estimated maximum purchase price could be positioned in a higher range, considering that there are examples of auctions that have reached significantly higher prices.

Estimated maximum purchase price: It could be reasonable to estimate a maximum purchase price around 200-250 euros, as this aligns with previous hammer prices and the demand trend. However, if the coin has high conservation, the price could be even higher.

Therefore, a prudent maximum purchase price could be 250 euros.