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

1. Current starting price: 1800 euros.
2. Average auction price: 1873.22 euros.
3. Auction price history:
- Previous auction prices varied, but there are several examples where the coin has sold for over 2000 euros, especially in higher grades (for example, 2400 euros and 2600 euros in grades 90 and 95).
4. Condition: There is no specific information about the condition of this coin, which may affect its value. However, considering that there have been successful sales in the range of 2000-2600 euros in the past, we can infer that the price could be in that range if the condition is acceptable.

Given that the starting price is 1800 euros and considering the auction price history, an estimated maximum purchase price could be between 2200 and 2600 euros, depending on the demand at the auction and the bidders' perception of the coin's condition.

Maximum purchase estimate: 2500 euros.