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

1. Current starting price: 2500 euros.
2. Average past starting price: 2650 euros.
3. Average past auction price: 3950 euros.
4. Auction history: The coin has been auctioned 3 times and has sold on all occasions, indicating consistent interest.
5. Condition values: Although there is no specific information about the condition of this coin, previous auction prices (2200 and 2900 euros) suggest that condition may be an important factor.

Given that the average auction price is significantly higher than the current starting price, and considering that the coin has a history of successful sales, it can be inferred that there is potential for price increase.

However, the lack of information about the condition may make buyers more cautious. Therefore, an estimated maximum purchase price could be positioned between the average auction price and the average starting price, adjusted for the uncertainty of the condition.

Estimated maximum purchase price: 3500 euros.

This price reflects a balance between historical interest in the coin and the lack of information about its current state.