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

1. Starting price: 300 euros.
2. Average auction price: 121.29 euros.
3. Auction price history:
- Previous auction prices varied significantly, with some reaching up to 390 euros in the case of unspecified condition.
- Most auction prices ranged between 40 and 300 euros, depending on the condition.

4. Condition: No information is provided about the current condition of the coin, making the estimation more complicated. However, by observing the auction prices of coins with similar conditions (around 70-90), we can infer that the price could be in a mid-range.

5. Analysis of previous prices:
- The coin has had successful auctions in the past, with prices ranging from 35 to 390 euros.
- The trend shows that coins with conditions of 70 to 90 have reached auction prices between 60 and 300 euros.

Given that there is no information about the current condition, and considering the starting price of 300 euros, an estimated maximum purchase price could be in the range of 200 to 300 euros, depending on demand and collector interest at the time of the auction.

Maximum purchase estimate: 250 euros. This is based on the analysis of previous prices and the lack of information about the current condition.